Radiation system

ABSTRACT

A radiation alteration device includes a continuously undulating reflective surface, wherein the shape of the continuously undulating reflective surface follows a substantially periodic pattern.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase entry of PCT patentapplication no. PCT/EP2016/053216, which was filed on Feb. 16, 2016,which claims priority to European patent application nos. EP 15157192.4,filed on Mar. 2, 2015, EP 15168832.2, filed on May 22, 2015, and EP15189676.8, filed on Oct. 14, 2015, each of which is incorporated hereinin its entirety by reference.

FIELD

The present invention relates to a radiation system. In particular, butnot exclusively, the radiation system may form part of a lithographicsystem comprising at least one lithographic apparatus.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may, for example, project a pattern from a patterning device(e.g. a mask) onto a layer of radiation-sensitive material (resist)provided on a substrate.

The wavelength of radiation used by a lithographic apparatus to projecta pattern onto a substrate determines the minimum size of features whichcan be formed on that substrate. A lithographic apparatus which uses EUVradiation, being electromagnetic radiation having a wavelength withinthe range 4-20 nm, may be used to form smaller features on a substratethan a conventional lithographic apparatus (which may for example useelectromagnetic radiation with a wavelength of 193 nm).

A lithographic apparatus may be provided with EUV radiation from aradiation system. It is an object of the present invention to obviate ormitigate at least one problem of prior art techniques.

SUMMARY

According a first aspect of the invention there is provided a radiationsystem comprising a beam splitting apparatus configured to receive amain radiation beam and split the main radiation beam into a pluralityof branch radiation beams and a radiation alteration device arranged toreceive an input radiation beam and output a modified radiation beam,wherein the radiation alteration device is configured to provide anoutput modified radiation beam which has an increased etendue, whencompared to the received input radiation beam, wherein the radiationalteration device is arranged such that the input radiation beam whichis received by the radiation alteration device is a main radiation beamand the radiation alteration device is configured to provide a modifiedmain radiation beam to the beam splitting apparatus, or wherein theradiation alteration device is arranged such that the input radiationbeam which is received by the radiation alteration device is a branchradiation beam output from the beam splitting apparatus.

The radiation alteration device may act as an optical diffuser and maydiffuse the input radiation beam. The radiation alteration device may bepositioned upstream or downstream of the beam splitting apparatus. Thatis, the radiation alteration device may alter a radiation beam prior toproviding the main radiation beam to a beam splitting apparatus or theradiation alteration device may alter a branch radiation beam which isformed at the beam splitting apparatus. Some embodiments may includemore than one radiation alteration device. For example a first radiationalteration device may be arranged to alter a radiation beam prior toproviding the radiation beam to the beam splitting apparatus and asecond radiation alteration device may be arranged to alter a branchradiation beam which is formed at the beam splitting apparatus. Someembodiments may include a radiation alteration device for each branchradiation beam.

The radiation alteration device may be configured to increase theetendue of the radiation beam such that the modified radiation beamwhich is output from the radiation alteration device has an etenduewhich is greater than at least 10 times the etendue of the inputradiation beam which is received by the radiation alteration device.

The radiation alteration device may be configured to increase theetendue of the EUV radiation beam such that the modified radiation beamwhich is output from the radiation alteration device has an etenduewhich is greater than at least 1×10⁴ times the etendue of the inputradiation beam which is received by the radiation alteration device.

The beam splitting apparatus may comprise a plurality of reflectivefacets, each reflective facet being arranged to receive a portion of themain radiation beam and reflect the portion of the main radiation beamso as to form a branch radiation beam.

The radiation alteration device may be configured to increase thespatial homogeneity of the intensity profile of the output modifiedradiation beam, when compared to the input received radiation beam.

The radiation system may further comprise one or more focussing opticsconfigured to form an image of the output modified radiation beam in afar field plane.

The radiation alteration device may be arranged such that the inputradiation beam which is received by the radiation alteration device is abranch radiation beam output from the beam splitting apparatus andwherein the one or more focussing optics are configured to form an imageof the output modified main radiation beam in a far field plane which issituated inside a lithographic apparatus.

The radiation alteration device may be arranged such that the inputradiation beam which is received by the radiation alteration device is amain radiation beam and the radiation alteration device is configured toprovide a modified main radiation beam to the beam splitting apparatusand wherein the one or more focussing optics are configured to form animage of the output modified main radiation beam in a far field planewhich is situated at or near to the beam splitting apparatus.

The radiation alteration device may comprise a tube having a firstopening arranged to receive the input radiation beam and a secondopening arranged to output the modified radiation beam, wherein the tubeis defined by a substantially reflective internal surface, and whereinthe internal surface is arranged so as to cause the radiation beam whichenters the tube through the first opening to undergo multiple successivereflections at the internal surface, thereby modifying the radiationbeam before the modified radiation beam exits the tube through thesecond opening.

The radiation alteration device may comprise a plurality of reflectivefacets each arranged to receive and reflect a portion of the inputradiation beam, so as to form a plurality of sub-beams reflected fromthe plurality of reflective facets and wherein the plurality ofreflective facets are arranged to direct the sub-beams to form theoutput modified radiation beam.

The radiation alteration device may comprise a first diffusing elementcomprising a first roughened reflective surface, a second diffusingelement comprising a second roughened reflective surface and one or moreactuators configured to move the first and/or second diffusing elementsso as to cause movement of the first and/or the second roughenedreflective surfaces, wherein the first roughened reflective surface isconfigured to receive the input radiation beam and reflect the radiationbeam so as to be incident on the second roughened reflective surface andwherein the second roughened reflective surface is arranged to reflectthe radiation beam received from the first roughened reflective surfaceso as to form the output modified radiation beam.

The radiation alteration device may comprise a continuously undulatingreflective surface arranged to receive and reflect the input radiationbeam, wherein the shape of the continuously undulating reflectivesurface follows a substantially periodic pattern.

The radiation system may further comprise a radiation source configuredto emit EUV radiation, wherein the main radiation beam comprises atleast a portion of the EUV radiation emitted by the radiation source.

The radiation source may comprise at least one free electron laser.

The radiation system may further comprise beam expanding opticsconfigured to expand the cross-section of the main radiation beam, priorto providing the main radiation beam to the beam splitting apparatus.

The radiation system may further comprise directing optics configured todirect at least one of the branch radiation beams to a respectivelithographic apparatus.

The radiation system may comprise a plurality of radiation alterationdevices and wherein each of the plurality of radiation alterationdevices is arranged such that an input radiation beam which is receivedby each of the radiation alteration devices is one of the branchradiation beams output from the beam splitting apparatus.

According to a second aspect of the invention there is provided aradiation system comprising a radiation source configured to emit EUVradiation and a radiation alteration device comprising a tube having afirst opening arranged to receive an EUV radiation beam and a secondopening arranged to output the EUV radiation beam, wherein the EUVradiation beam which enters the radiation alteration device comprises atleast a portion of the EUV radiation emitted by the radiation source,wherein the tube is defined by an internal surface which issubstantially reflective to EUV radiation, and wherein the internalsurface is arranged so as to cause the EUV radiation beam which entersthe tube through the first opening to undergo multiple successivereflections at the internal surface, thereby modifying the EUV radiationbeam before the EUV radiation beam exits the tube through the secondopening.

The radiation alteration device advantageously modifies the EUVradiation beam. The EUV radiation beam which is output from theradiation alteration device may, for example, be provided to alithographic apparatus. The radiation alteration device may modify theEUV radiation beam such that the radiation beam which is output from theradiation alteration device provides advantageous effects when providedto a lithographic apparatus. Alternatively the EUV radiation beam whichis output from the radiation alteration device may be provided to a beamsplitting apparatus which is configured to split the EUV radiation beaminto a plurality of branch radiation beams.

The tube may include a bend.

The bend may advantageously increase the range of grazing angles atwhich the EUV radiation is incident on the internal surface of the tube.The bend may be a stepped bend or may be a continuous bend such that thetube is curved.

A bend angle of the bend in the tube may be less than about 5 degrees.

The amount of radiation which is absorbed during reflection of the EUVradiation at the internal surface may increase with increases in thegrazing angle with which the EUV radiation is incident on the internalsurface. Restricting the bend angle to less than about 5 degrees mayadvantageously limit the grazing angle with which the EUV radiation isincident on the internal surface and may therefore limit the amount ofradiation which is absorbed during reflection of the EUV radiation atthe internal surface.

The bend in the tube may be configured such that there is no direct lineof sight through the radiation alteration device.

The cross-sectional shape of the internal surface of the tube may be apolygon.

The cross-sectional shape of the internal surface of the tube may be arectangle.

The cross-sectional shape of the internal surface of the tube may be asquare.

The cross-sectional shape of the internal surface of the tube may be ahexagon.

The tube may comprise a first section and a second section arranged toreceive EUV radiation from the first section, wherein the first andsecond sections are arranged to form a gap between the first and secondsections through which gas may enter or leave the tube.

Gas which enters and leaves the tube may advantageously serve to cleancontamination from the internal surface of the tube.

The first and second sections may be arranged such that EUV radiationwhich enters the tube through the first opening does not exit the tubethrough the gap.

Preventing radiation from exiting the tube through the gap prevents lossof radiation from the EUV radiation beam through the gap.

The radiation system may further comprise a gas supply configured toinject gas into the tube through the gap.

The gas supply may be configured to inject hydrogen into the tubethrough the gap.

The radiation system may further comprise an actuator operable toactuate the tube so as to cause the tube to undergo an oscillatorymotion.

Forcing the tube to undergo an oscillatory motion advantageously causestemporal scrambling of the radiation which is output from the radiationalteration device.

The actuator may be operable to actuate the tube so as to cause the tubeto undergo an oscillatory motion having a frequency which is greaterthan about 1 kHz.

The radiation system may further comprise one or more optical elementsarranged to receive EUV radiation emitted from the radiation source anddirect the EUV radiation beam, formed of at least a portion of the EUVradiation emitted by the radiation source, to enter the radiationalteration device through the first opening of the radiation alterationdevice.

The one or more optical elements may include at least one focusing opticconfigured to alter the divergence of the EUV radiation beam such thatthe EUV radiation beam which enters the radiation alteration device hasa non-zero divergence.

Altering the divergence of the EUV radiation beam such that the EUVradiation beam enters the radiation alteration device having a non-zerodivergence ensures that at least some of the EUV radiation is incidenton the internal surface of the radiation alteration device and undergoesreflection at the internal surface. The EUV radiation beam may enter theradiation alteration device having a positive divergence such that theEUV radiation diverges towards the internal surface. Alternatively theEUV radiation beam may enter the radiation alteration device having anegative divergence such that the radiation beam is focussed to a focalpoint which is situated inside the radiation alteration device. The EUVradiation beam may then have a positive divergence after the focal pointsuch that the EUV radiation diverges towards the internal surface.

The radiation alteration device may have a length L between the firstand second openings, the second opening may have a diameter D and the atleast one focusing optic may be configured to alter the divergence ofthe EUV radiation beam such that the EUV radiation beam is provided tothe radiation alteration device with a half divergence θ. The radiationalteration device and the at least one focusing optic may be configuredsuch that θL/D is greater than or equal to about 10.

Configuring the radiation alteration device and the at least onefocusing optic such that θL/D is greater than or equal to about 10 mayadvantageously ensure that the homogeneity of a spatial intensityprofile of the EUV radiation beam is increased by a desirable amount forusing the modified EUV radiation beam in a lithographic apparatus.

A half divergence of the EUV radiation beam may be less than about 100milliradians.

A half divergence of the EUV radiation beam may be less than about 10milliradians.

The at least one focusing optic may be configured to focus the EUVradiation beam to a focal point such that the EUV radiation beam isprovided to the radiation alteration device with a positive divergence.

The tube may define an optical axis which extends along thecross-sectional centre of the tube and extends into and out of the tubethrough the first and second openings. The at least one focusing opticmay be configured to focus the EUV radiation beam to a focal point whichdoes not lie on the optical axis.

Focusing the EUV radiation beam to a focal point which does not lie onthe optical axis may advantageously increase the range of grazing anglesat which EUV radiation is incident on the internal surface.

A line extending from the focal point to a position on the optical axis,at which the optical axis passes through the first opening, may form anoff-axis angle with the optical axis. The off-axis angle may beapproximately the same as or greater than a half-divergence of the EUVradiation beam.

The radiation alteration device may be configured to increase theetendue of the EUV radiation beam such that the EUV radiation beam whichexits the radiation alteration device through the second opening of theradiation alteration device has an etendue which is greater than theetendue of the EUV radiation beam which enters the first opening of theradiation alteration device.

The radiation alteration device may be configured to increase theetendue of the EUV radiation beam such that the EUV radiation beam whichexits the radiation alteration device through the second opening of theradiation alteration device has an etendue which is greater than atleast 10 times the etendue of the EUV radiation beam which enters thefirst opening of the radiation alteration device.

The radiation alteration device may be configured to increase theetendue of the EUV radiation beam such that the EUV radiation beam whichexits the radiation alteration device through the second opening of theradiation alteration device has an etendue which is greater than atleast 1×10⁴ times the etendue of the EUV radiation beam which enters thefirst opening of the radiation alteration device.

The radiation alteration device may be configured to provide a mappingof rays which form the EUV radiation beam from a first plane situateddownstream of the first opening of the radiation alteration device to asecond plane situated upstream of the second opening of the radiationalteration device, wherein the mapping serves to substantially scramblethe cross-sectional position of the rays between the first plane and thesecond plane.

The radiation alteration device may be configured to reduce the spatialcoherence of the EUV radiation beam.

Reducing the spatial coherence of the EUV radiation beam mayadvantageously prevent interference patterns (e.g. a speckle pattern)from forming in the EUV radiation beam.

The at least one focusing optic and the radiation alteration device maybe configured such that different portions of the EUV radiation beampropagate along optical paths through the radiation alteration devicehaving different path lengths, and wherein the range of different pathlengths along which different portions of the EUV radiation beampropagate is greater than the coherence length of the EUV radiationbeam.

The radiation alteration device may be configured to increase thespatial homogeneity of the intensity profile of the EUV radiation beam,such that the EUV radiation beam which exits the radiation alterationdevice through the second opening has a spatial intensity distributionwhich is more homogenous than the spatial intensity profile of the EUVradiation beam which enters the first opening of the radiationalteration device.

The radiation system may further comprise a beam splitting apparatusconfigured to receive the EUV radiation beam which exits the secondopening of the radiation alteration device and split the EUV radiationbeam into a plurality of branch radiation beams.

The beam splitting apparatus may comprise a plurality of reflectivefacets arranged to receive different portions of the cross-section ofthe EUV radiation beam which exits the second opening of the radiationalteration device and reflect the different portions of thecross-section in different directions.

The radiation system may further comprise at least one focusing opticconfigured to magnify the radiation beam which exits the second openingof the radiation alteration device onto the beam splitting apparatus,the magnification being such that the divergence of the radiation beamwhich is incident on the beam splitting apparatus is less than thedivergence of the radiation beam which is output from the second openingof the radiation alteration device.

The radiation source may comprise at least one free electron laseroperable to emit EUV radiation.

The radiation source may comprise a first free electron laser, a secondfree electron laser and a beam combination apparatus configured tocombine EUV radiation emitted from the first free electron laser withradiation emitted from the second free electron laser to form a combinedradiation beam, the combined radiation beam having a cross-section whichincludes a first portion formed from radiation emitted from the firstfree electron laser and a second portion formed from radiation emittedfrom the second free electron laser.

The radiation alteration device may be configured to receive thecombined radiation beam and spatially scramble the combined radiationbeam to form a scrambled combined radiation beam, the spatial scramblingbeing such that the spatially scrambled combined radiation beam includesoverlap between radiation emitted from the first free electron laser andradiation emitted from the second free electron laser.

The spatial scrambling may be such that a variation in the power ofradiation emitted by one or more of the first and second free electronlasers does not cause a substantial variation in a spatial distributionof power in the scrambled combined radiation beam which exits theradiation alteration device.

The radiation system may further comprise a beam splitting apparatusconfigured to receive a main radiation beam, wherein the main radiationbeam comprises at least a portion of the EUV radiation emitted from theradiation source and wherein the beam splitting apparatus comprises aplurality of reflective facets arranged to receive different portions ofthe cross-section of the main radiation beam and reflect the differentportions of the cross-section in different directions so as to form aplurality of branch radiation beams. The radiation alteration device maybe arranged to receive one of the branch radiation beams through thefirst opening of the radiation alteration device.

The reflective facets may be arranged to receive different sectors ofthe cross-section of the main radiation beam and reflect the differentsectors in different directions so as to split the main radiation beaminto the plurality of branch radiation beams.

The radiation system may further comprise at least one focusing opticconfigured to focus the EUV radiation beam which exits the radiationalteration device to an intermediate focus.

The at least one focusing optic may be configured to focus the EUVradiation beam so as to form an image of the second opening of theradiation alteration device on a far-field plane disposed downstream ofthe intermediate focus.

The at least one focusing optic may comprise a first focusing opticconfigured to form an image of the second opening of the radiationalteration device at an image plane and a second focusing opticconfigured to form an image of the image plane on the far-field plane.

The first focusing optic may have a positive focusing power.

The second focusing optic may have a positive focusing power.

The first focussing optic and/or the second focussing optic may comprisea first reflective element and a second reflective element.

The first focusing optic and/or the second focusing optic may comprise aWolter telescope.

The Wolter telescope may comprise a type-III Wolter telescope.

According to a third aspect of the invention there is provided aradiation system comprising a radiation source configured to emit EUVradiation and a radiation alteration device arranged to receive an EUVradiation beam comprising at least a portion of the EUV radiationemitted by the radiation source wherein the radiation alteration devicecomprises a plurality of reflective facets each arranged to receive andreflect a portion of the EUV radiation beam, so as to form a pluralityof sub-beams reflected from the plurality of reflective facets andwherein the plurality of reflective facets are arranged to direct thesub-beams to form a modified EUV radiation beam, wherein the radiationalteration device is configured to provide the modified radiation beamto at least one lithographic apparatus.

The radiation alteration device advantageously modifies the EUVradiation beam. The EUV radiation beam which is output from theradiation alteration device may, for example, be provided to alithographic apparatus. The radiation alteration device may modify theEUV radiation beam such that the radiation beam which is output from theradiation alteration device provides advantageous effects when providedto a lithographic apparatus. Alternatively the EUV radiation beam whichis output from the radiation alteration device may be provided to a beamsplitting apparatus which is configured to split the EUV radiation beaminto a plurality of branch radiation beams.

The modified EUV radiation beam which is formed by the radiationalteration may be equivalent to radiation emitted from a plurality ofpoint sources. When viewed in a far-field location, the modified EUVradiation beam may be equivalent to radiation emitted from a planarhigh-etendue light source. The modified EUV radiation beamadvantageously has a higher etendue than the etendue of the EUVradiation beam which is incident on the radiation alteration device. Theradiation alteration device also serves to increase the homogeneity of aspatial intensity profile of the EUV radiation beam. That is, thespatial intensity profile of the modified EUV radiation beam in a farfield location is more homogenous than the spatial intensity profile ofthe EUV radiation beam which is incident on the radiation alterationdevice.

The reflective facets may be configured to focus the sub-beams.

The reflective facets may comprise concave reflective surfaces.

The plurality of reflective facets may be arranged to focus thesub-beams to a plurality of focal points.

The plurality of focal points may lie in a plane of focal points.

The plurality of reflective facets may generally lie in a plane and theplane of focal points may be substantially parallel with the plane inwhich the reflective facets generally lie.

Each sub-beam may have an optical axis and the plane of focal points maybe substantially perpendicular to the optical axes of the sub-beams.

The focal points may be substantially uniformly spaced in the plane offocal points.

Each of the reflective facets may have a focal length which is greaterthan a length of the radiation alteration device.

Each of the reflective facets may have a generally rectangularcross-section.

Each of the reflective facets may have a generally hexagonalcross-section.

The reflective facets may be arranged in a honeycomb lattice.

The reflective facets may be configured to direct the sub-beams suchthat the sub-beams overlap with each other in a far field plane.

The reflective facets may be configured to direct the sub-beams suchthat the sub-beams illuminated substantially the same area in the farfield plane.

The reflective facets may have a concave shape.

The reflective facets may have a convex shape.

The radiation alteration device may comprise 16 or more reflectivefacets.

The radiation alteration device may comprises 64 or more reflectivefacets.

The radiation system may further comprise a beam splitting apparatusconfigured to receive the EUV radiation beam and split the EUV radiationbeam into a plurality of branch radiation beams.

The beam splitting apparatus may be configured to split the EUVradiation emitted from the radiation source into a plurality of branchradiation beams and the radiation alteration device may be configured toreceive and modify a branch radiation beam.

The radiation alteration device may be arranged to receive and modifythe radiation emitted from the radiation source and provide the modifiedEUV radiation beam to the beam splitting apparatus.

The radiation system may further comprise a beam expanding opticsconfigured to receive the EUV radiation beam from the radiation source,expand the cross-section of the radiation beam and provide the expandedradiation beam to the radiation alteration device.

The radiation system may further comprise directing optics, thedirecting optics being configured to receive a branch radiation beamfrom the beam splitting apparatus and direct the branch radiation beamto a lithographic apparatus.

According to a fourth aspect of the invention there is provided aradiation alteration device comprising a first diffusing elementcomprising a first roughened reflective surface, a second diffusingelement comprising a second roughened reflective surface, and one ormore actuators configured to move the first and/or second diffusingelements so as to cause movement of the first and/or the secondroughened reflective surfaces, wherein the first roughened reflectivesurface is configured to receive a radiation beam and reflect theradiation beam so as to be incident on the second roughened reflectivesurface and wherein the second roughened reflective surface is arrangedto reflect the radiation beam received from the first roughenedreflective surface so as to form a modified radiation beam.

The one or more actuators may be configured to rotate the first and/orthe second diffusing elements so as to cause rotation of the firstand/or the second roughened reflective surfaces.

The one or more actuators may be configured to cause the first and/orthe second roughened reflective surfaces to move at a speed of about 1meter per second or more.

The second roughened reflective surface may be arranged approximatelyperpendicular to the first roughened reflective surface.

The first and/or the second roughened reflective surfaces may bearranged to receive radiation at a grazing incidence angle of about 5degrees or less.

The first and/or the second roughened reflective surfaces may includeindentations which cause the roughened reflective surface to deviatefrom a flat plane.

A maximum angle which the first and/or the second roughened reflectivesurfaces form with the flat plane, may be less than or equal to about 10milliradians.

The first and second roughened reflective surfaces may each serve tointroduce an angular spread into the radiation beam.

According to a fifth aspect of the invention there is provided aradiation alteration device comprising a continuously undulatingreflective surface, wherein the shape of the continuously undulatingreflective surface follows a substantially periodic pattern.

The shape of the continuously undulating reflective surface may follow asubstantially periodic pattern in two perpendicular directions.

A unit cell of the periodic undulating reflective surface may comprise:a first portion having a substantially convex shape; a second portionhaving a substantially concave shape; a third portion having asubstantially saddle shape; and a fourth portion having a substantiallysaddle shape.

The unit cell may comprise a single period of the periodic pattern in afirst direction and a single period of the periodic pattern in a seconddirection perpendicular to the first direction.

The reflective surface may be shaped such within at least one of thefirst, second, third and fourth portions, the curvature of thereflective surface is substantially the same throughout the respectiveportion.

The reflective surface may be shaped such that within at least one ofthe first, second, third and fourth portions, the curvature of thereflective surface is different at different positions in the respectiveportion.

The reflective surface may be configured to receive a radiation beam andreflect the radiation beam so as to form a modified radiation beam andwherein, the reflective surface is shaped such that the modifiedradiation beam has an intensity distribution in a far field plane, theintensity distribution comprising an intensity maximum, wherein theintensity distribution decreases with increasing radial distance fromthe intensity maximum.

According to a sixth aspect of the invention there is provided aradiation system comprising a radiation source configured to emit EUVradiation, and a radiation alteration device according to the fifth orsixth aspect configured to receive a radiation beam comprising at leasta portion of the EUV radiation emitted by the radiation source.

According to a seventh aspect of the invention there is provided aradiation system comprising a radiation source configured to emit EUVradiation, a first radiation alteration device configured to receive amain radiation beam and output a modified main radiation beam, whereinthe main radiation beam comprises at least a portion of the EUVradiation emitted from the radiation source, a beam splitting apparatuscomprising a plurality of reflective facets arranged to receivedifferent portions of the cross-section of the main radiation beam andreflect the different portions of the cross-section in differentdirections so as to form a plurality of branch radiation beams and asecond radiation alteration device configured to receive a branchradiation beam and output a modified branch radiation beam, wherein thefirst and second radiation alteration devices each comprise: a tubehaving a first opening arranged to receive EUV radiation and a secondopening arranged to output EUV radiation and wherein the tube is definedby an internal surface which is substantially reflective to EUVradiation, and wherein the internal surface is arranged so as to causethe EUV radiation which enters the tube through the first opening toundergo multiple successive reflections at the internal surface, therebymodifying the EUV radiation before the EUV radiation exits the tubethrough the second opening; or a plurality of reflective facets eacharranged to receive and reflect a portion of the EUV radiation, so as toform a plurality of sub-beams reflected from the plurality of reflectivefacets and wherein the plurality of reflective facets are arranged todirect the sub-beams to form a modified EUV radiation, or a firstdiffusing element comprising a first roughened reflective surface, asecond diffusing element comprising a second roughened reflectivesurface and one or more actuators configured to move the first and/orsecond diffusing elements so as to cause movement of the first and/orthe second roughened reflective surfaces, wherein the first roughenedreflective surface is configured to receive an EUV radiation beam andreflect the EUV radiation beam so as to be incident on the secondroughened reflective surface and wherein the second roughened reflectivesurface is arranged to reflect the EUV radiation beam received from thefirst roughened reflective surface so as to form a modified radiationbeam.

The first and/or the second radiation alteration devices may include anyof the features of the radiation alteration device according to any ofthe other aspects.

According to an eighth aspect of the invention there is provided alithographic system comprising a radiation system according to any ofthe first to third aspects or the sixth or seventh aspects and alithographic apparatus arranged to receive at least a portion of the EUVradiation beam which exits a radiation alteration device.

The lithographic apparatus may include an illumination system configuredto condition at least a portion of the EUV radiation beam which exitsthe radiation alteration device, the illumination system including afacet mirror. The radiation system may comprise at least one focusingoptic configured to focus the EUV radiation beam which is provided tothe lithographic apparatus so as to form an image of the radiation beamoutput from the radiation alteration device onto the facet mirror,wherein the facet mirror comprises a plurality of reflective facets.

According to a ninth aspect of the invention there is provided aradiation alteration device suitable for use in a radiation systemaccording to any of the first to third aspects or the fifth or sixthaspects or a lithographic system according to the eighth aspect.

According to a tenth aspect of the invention there is provided a beamsplitting apparatus suitable for receiving a main radiation beam, thebeam splitting apparatus comprising a plurality of reflective facets,wherein the reflective facets are arranged to receive different sectorsof the cross-section of the main radiation beam and reflect thedifferent sectors in different directions so as to split the mainradiation beam into a plurality of branch radiation beams.

The beam splitting apparatus may further comprise one or more opticalelements configured to combine branch radiation beams which correspondto radially opposite sectors of the main radiation beam so as to form acombined branch radiation beam.

The beam splitting apparatus may further comprise one or more focussingoptics configured to receive the main radiation beam and expand the mainradiation beam to form an annular ring of radiation, wherein thereflective facets are arranged to receive different sectors of theannular ring of radiation.

According to an eleventh aspect of the invention there is provided abeam splitting apparatus suitable for receiving a main radiation beam,the beam splitting apparatus comprising a plurality of groups ofreflective facets, wherein each group of reflective facets comprisesreflective facets arranged to receive different portions of the mainradiation beam and reflect the different received portions to form abranch radiation beam comprising a combination of the differentreflected portions, the plurality of groups of reflective facets therebyeach forming a different branch radiation beam.

The beam splitting apparatus may be configured to receive a mainradiation beam propagating substantially along a beam axis and whereinthe reflective facets which form a group of reflective facets aresituated at substantially the same location along the beam axis.

The groups of reflective facets may be situated at substantially thesame location along the beam axis.

The reflective facets from different groups of reflective facets may bearranged adjacent to each other.

The adjacent reflective facets may be in contact with each other.

The beam splitting apparatus may be configured to receive a mainradiation beam propagating substantially along a beam axis and wherein aplurality of reflective facets which form a group of reflective facetsare separated from each other in a direction substantially perpendicularto the beam axis.

Each reflective facet of a group of reflective facets may comprise areflective surface having substantially the same orientation.

The plurality of groups of reflective facets may comprise a first groupof reflective facets and a second group of reflective facets wherein thearrangement of the second group of reflective facets is substantiallythe same as a rotation of the first group of reflective facets.

Each group of reflective facets may comprise an arrangement ofreflective facets which is substantially the same as a rotation of anarrangement of reflective facets which forms another of the groups ofreflective facets.

The beam splitting apparatus may be configured to receive a mainradiation beam propagating substantially along a beam axis and whereinthe rotation is a rotation substantially about the beam axis.

According to a twelfth aspect of the invention there is provided anoptical system comprising a beam splitting apparatus according to theseventh aspect and a radiation alteration device configured to receive abranch radiation beam formed by the beam splitting apparatus, whereinthe radiation alteration device comprises: a tube having a first openingarranged to receive the branch radiation beam and a second openingarranged to output the branch radiation beam, wherein the tube isdefined by an internal surface which is substantially reflective, andwherein the internal surface is arranged so as to cause the branchradiation beam which enters the tube through the first opening toundergo multiple successive reflections at the internal surface, therebymodifying the branch radiation beam before the branch radiation beamexits the tube through the second opening; or a plurality of reflectivefacets each arranged to receive and reflect a portion of the branchradiation beam, so as to form a plurality of sub-beams reflected fromthe plurality of reflective facets and wherein the plurality ofreflective facets are arranged to direct the sub-beams to form amodified branch radiation beam, or a first diffusing element comprisinga first roughened reflective surface, a second diffusing elementcomprising a second roughened reflective surface and one or moreactuators configured to move the first and/or second diffusing elementsso as to cause movement of the first and/or the second roughenedreflective surfaces, wherein the first roughened reflective surface isconfigured to receive an EUV radiation beam and reflect the EUVradiation beam so as to be incident on the second roughened reflectivesurface and wherein the second roughened reflective surface is arrangedto reflect the EUV radiation beam received from the first roughenedreflective surface so as to form a modified radiation beam.

According to a thirteenth aspect of the invention there is provided amethod of modifying an EUV radiation beam, the method comprisingemitting EUV radiation from a radiation source, forming an EUV radiationbeam comprising at least a portion of the EUV radiation emitted by theradiation source, directing the EUV radiation beam to enter a radiationalteration device comprising a tube having a first opening arranged toreceive the EUV radiation beam and a second opening arranged to outputthe EUV radiation beam, wherein the tube is defined by an internalsurface which is substantially reflective to EUV radiation, and whereinthe internal surface is arranged so as to cause the EUV radiation beamwhich enters the tube through the first opening to undergo multiplesuccessive reflections at the internal surface, thereby modifying theEUV radiation beam before the EUV radiation beam exits the tube throughthe second opening.

According to a fourteenth aspect of the invention there is provided amethod of modifying an EUV radiation beam, the method comprisingemitting EUV radiation from a radiation source, forming an EUV radiationbeam comprising at least a portion of the EUV radiation emitted by theradiation source, directing the EUV radiation beam to be incident on aradiation alteration device comprising a plurality of reflective facetseach arranged to receive and reflect a portion of the EUV radiationbeam, so as to form a plurality of sub-beams reflected from theplurality of reflective facets and wherein the plurality of reflectivefacets are arranged to direct the sub-beams to form a modified EUVradiation beam and providing the modified EUV radiation beam to alithographic apparatus.

According to a fifteenth aspect of the invention there is provided amethod of modifying a radiation beam, the method comprising reflectingthe radiation beam at a first diffusing element comprising a firstroughened reflective surface, reflecting the radiation beam, which isreflected at the first diffusing element, at a second diffusing elementcomprising a second roughened reflective surface and moving the firstand/or second diffusing elements so as to cause movement of the firstand/or the second roughened reflective surfaces.

According to a sixteenth aspect of the invention there is provided amethod of forming a roughened reflective surface the method comprisingforming indentations in a surface of a substrate, wherein theindentations cause the surface of the substrate to vary from a flatplane and disposing a reflective coating on the surface of thesubstrate.

Forming indentations in the surface of the substrate may compriseabrasive blasting of the surface of the substrate.

The method may further comprise electropolishing the surface of thesubstrate prior to disposing the reflective coating on the surface ofthe substrate.

The method may further comprise electropolishing the reflective coating.

According to a seventeenth aspect of the invention there is provided amethod of forming a roughened reflective surface the method comprisingforming a patterned substrate, deforming a metal sheet using thepatterned substrate, deforming a surface of a substrate using the metalsheet as a mandrel and disposing a reflective coating on the deformedsurface of the substrate.

The patterned substrate may comprise a plurality of protrusionsextending out of an otherwise substantially flat surface.

Deforming a metal sheet using the patterned substrate may compriseperforming a hydraulic forming process.

Deforming a surface of a substrate using the metal sheet as a mandrel,may comprise performing an electroforming process.

The substrate may be formed from metal.

The metal may comprise at least one of nickel, copper and aluminium.

The reflective coating may comprise at least one of ruthenium andmolybdenum.

Various aspects and features of the invention set out above or below maybe combined with various other aspects and features of the invention aswill be readily apparent to the skilled person.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 is a schematic illustration of a lithographic system comprising afree electron laser according to an embodiment of the invention;

FIG. 2 is a schematic illustration of a lithographic apparatus thatforms part of the lithographic system of FIG. 1;

FIG. 3 is a schematic illustration of a free electron laser that formspart of the lithographic system of FIG. 1;

FIG. 4 is a schematic illustration of a portion of a lithographic systemaccording to an embodiment of the invention;

FIG. 5 is a schematic illustration of an embodiment of a radiationalteration device;

FIG. 6 is a schematic representation of radiation which may be input tothe radiation alteration device of FIG. 5;

FIG. 7 is a schematic representation of radiation which may be outputfrom the radiation alteration device of FIG. 5;

FIGS. 8A-8C are schematic illustrations of an alternative embodiment ofa radiation alteration device and the effect that the radiationalteration device has on radiation passing through the radiationalteration device;

FIG. 9 is a schematic illustration of a radiation beam being focused toan off-axis position before entering a radiation alteration device;

FIG. 10 is a schematic illustration of an alternative embodiment of aradiation alteration device which includes a gas supply;

FIG. 11 is a schematic illustration of an alternative embodiment of aradiation alteration device which includes an actuator configured tocause the radiation device to undergo an oscillatory motion;

FIGS. 12A-12E are schematic illustrations of focusing schemes which maybe used to focus radiation which is output from a radiation alterationdevice;

FIG. 13 is a schematic illustration of a radiation alteration deviceprovided upstream of a beam splitting apparatus;

FIGS. 14A and 14B are schematic illustrations of embodiments of a beamsplitting apparatus which may be used in combination with a radiationalteration device;

FIG. 15 is a schematic illustration of a radiation alteration device, abeam splitting apparatus and a focusing optic configured to form animage of radiation which is output from the radiation alteration deviceat the beam splitting apparatus;

FIG. 16 is a schematic illustration of an arrangement of free electronlasers arranged to form combined radiation beams;

FIG. 17 is a schematic illustration of a cross-section of a combinedradiation beam formed by the arrangement of free electron lasers of FIG.16 at an input to a radiation alteration device;

FIG. 18 is a schematic illustration of a first portion of an embodimentof a beam splitting apparatus;

FIG. 19 is a schematic illustration of the first portion of the beamsplitting apparatus of FIG. 18 in combination with a second portion ofthe beam splitting apparatus;

FIGS. 20A and 20B are schematic illustrations of a radiation beam whichis incident on the beam splitting apparatus of FIG. 19;

FIG. 21 is a schematic illustration of an arrangement of mirrors whichmay be used to combine branch radiation beams formed by a beam splittingapparatus;

FIGS. 22A and 22B are schematic illustrations of an arrangement oflenses which may be used to form an annular ring of radiation to beprovided to a beam splitting apparatus and an annular ring of radiationwhich is incident on a beam splitting apparatus; and

FIG. 23 is a schematic illustration of a beam splitting apparatus whichsplits an annular ring of radiation into a plurality of branch radiationbeams;

FIG. 24 is a schematic illustration of an embodiment of radiationalteration device comprising a plurality of reflective facets;

FIG. 25 is a schematic illustration of a perspective view of a radiationalteration device comprising a plurality of reflective facets;

FIG. 26 is a schematic illustration of a perspective view of analternative embodiment of a radiation alteration device comprising aplurality of reflective facets;

FIG. 27 is a schematic illustration of a further alternative embodimentof a radiation alteration device comprising a plurality of reflectivefacets;

FIG. 28 is a schematic illustration of an embodiment of a radiationalteration device comprising a plurality of reflective facets as viewedfrom above;

FIG. 29 is a schematic illustration of an alternative embodiment of aradiation alteration device comprising a plurality of reflective facets;

FIG. 30 is a schematic illustration of a lithographic system accordingto an embodiment of the invention;

FIG. 31 is a schematic illustration of beam expanding optics of thelithographic system of FIG. 30;

FIG. 32 is a schematic representation of optical elements which form anembodiment of directing optics of the lithographic system of FIG. 30;

FIG. 33 is a schematic illustration of a portion of a beam splittingapparatus of the lithographic system of FIG. 30;

FIG. 34 is a schematic illustration of a radiation alteration deviceaccording to an embodiment of the invention;

FIG. 35 is a schematic illustration of a roughened reflective surfacewhich forms part of the radiation alteration device of FIG. 34;

FIG. 36 is a schematic illustration of a diffusing element which formspart of the radiation alteration device of FIG. 34;

FIGS. 37A-37E are schematic illustrations of steps of a method offorming a roughened reflective surface according to an embodiment of theinvention;

FIGS. 38A and 38B are schematic representations of focusing schemeswhich may be used to image an angular intensity profile of a modifiedradiation beam output from a radiation alteration device onto a farfield plane;

FIG. 39 is a schematic illustration of a radiation alteration deviceaccording to an alternative embodiment of the invention;

FIG. 40 is a schematic illustration of a unit cell of the radiationalteration device of FIG. 39;

FIGS. 41A and 41B are schematic representations of functions which maydefine the surface of the radiation alteration device of FIG. 39;

FIG. 42 is a schematic illustration of a radiation alteration deviceaccording to an embodiment of the invention;

FIG. 43 is a schematic representation of an intensity profile which maybe formed by the radiation alteration device of FIG. 42;

FIGS. 44A and 44B are schematic representations of focusing schemeswhich may be used to image a modified radiation beam output from aradiation alteration device;

FIG. 45 is a schematic representation of an alternative embodiment of alithographic apparatus;

FIG. 46 is a schematic illustration of a beam splitting apparatusaccording to an embodiment of the invention;

FIGS. 47A-47C are schematic illustrations of alternative embodiments ofa beam splitting apparatuses according to the invention; and

FIG. 48 is a schematic illustration of further alternative embodimentsof a beam splitting apparatuses according to the invention.

DETAILED DESCRIPTION

FIG. 1 shows a lithographic system LS according to one embodiment of theinvention. The lithographic system LS comprises a radiation source SO, abeam delivery system BDS and a plurality of lithographic apparatusLA_(a)-LA_(n) (e.g. eight lithographic apparatus). The radiation sourceSO is configured to generate an extreme ultraviolet (EUV) radiation beamB (which may be referred to as a main beam). The radiation source SO andthe beam delivery system BDS may together be considered to form aradiation system, the radiation system being configured to provideradiation to one or more lithographic apparatus LA_(a)-LA_(n).

The beam delivery system BDS comprises beam splitting optics and mayoptionally also comprise beam expanding optics and/or beam shapingoptics. The main radiation beam B is split into a plurality of radiationbeams B_(a)-B_(n) (which may be referred to as branch beams), each ofwhich is directed to a different one of the lithographic apparatusLA_(a)-LA_(n), by the beam delivery system BDS.

In an embodiment, the branch radiation beams B_(a)-B_(n) are eachdirected through a respective attenuator (not shown in FIG. 1). Eachattenuator may be arranged to adjust the intensity of a respectivebranch radiation beam B_(a)-B_(n) before the branch radiation beamB_(a)-B_(n) passes into its corresponding lithographic apparatusLA_(a)-LA_(n).

The radiation source SO, beam delivery system BDS and lithographicapparatus LA_(a)-LA_(n) may all be constructed and arranged such thatthey can be isolated from the external environment. A vacuum may beprovided in at least part of the radiation source SO, beam deliverysystem BDS and lithographic apparatuses LA_(a)-LA_(n) so as to reducethe absorption of EUV radiation. Different parts of the lithographicsystem LS may be provided with vacuums at different pressures (i.e. heldat different pressures which are below atmospheric pressure).

Referring to FIG. 2, a lithographic apparatus LA_(a) comprises anillumination system IL, a support structure MT configured to support apatterning device MA (e.g. a mask), a projection system PS and asubstrate table WT configured to support a substrate W. The illuminationsystem IL is configured to condition the branch radiation beam B_(a)that is received by that lithographic apparatus LA_(a) before it isincident upon the patterning device MA. The projection system PS isconfigured to project the radiation beam B_(a)′ (now patterned by thepatterning device MA) onto the substrate W. The substrate W may includepreviously formed patterns. Where this is the case, the lithographicapparatus aligns the patterned radiation beam B_(a)′ with a patternpreviously formed on the substrate W.

The branch radiation beam B_(a) that is received by the lithographicapparatus LA_(a) passes into the illumination system IL from the beamdelivery system BDS though an opening 8 in an enclosing structure of theillumination system IL. Optionally, the branch radiation beam B_(a) maybe focused to form an intermediate focus at or near to the opening 8.

The illumination system IL may include a field facet mirror 10 and apupil facet mirror 11. The field facet mirror 10 and pupil facet mirror11 together provide the radiation beam B_(a) with a desiredcross-sectional shape and a desired angular distribution. The radiationbeam B_(a) passes from the illumination system IL and is incident uponthe patterning device MA held by the support structure MT. Thepatterning device MA reflects and patterns the radiation beam to form apatterned beam B_(a)′. The illumination system IL may include othermirrors or devices in addition to or instead of the field facet mirror10 and pupil facet mirror 11. The illumination system IL may, forexample, include an array of independently moveable mirrors. Theindependently moveable mirrors may, for example, measure less than 1 mmacross. The independently moveable mirrors may, for example, bemicroelectromechanical systems (MEMS) devices.

Following redirection (e.g. reflection) from the patterning device MAthe patterned radiation beam B_(a)′ enters the projection system PS. Theprojection system PS comprises a plurality of mirrors 13, 14 which areconfigured to project the radiation beam B_(a)′ onto a substrate W heldby the substrate table WT. The projection system PS may apply areduction factor to the radiation beam, forming an image with featuresthat are smaller than corresponding features on the patterning deviceMA. A reduction factor of 4 may, for example, be applied. Although theprojection system PS has two mirrors in FIG. 2, the projection systemmay include any number of mirrors (e.g. six mirrors).

The lithographic apparatus LA_(a) is operable to impart a radiation beamB_(a) with a pattern in its cross-section and project the patternedradiation beam onto a target portion of a substrate thereby exposing atarget portion of the substrate to the patterned radiation. Thelithographic apparatus LA_(a) may, for example, be used in a scan mode,wherein the support structure MT and the substrate table WT are scannedsynchronously while a pattern imparted to the radiation beam B_(a)′ isprojected onto a substrate W (i.e. a dynamic exposure). The velocity anddirection of the substrate table WT relative to the support structure MTmay be determined by the demagnification and image reversalcharacteristics of the projection system PS.

Referring again to FIG. 1, the radiation source SO is configured togenerate an EUV radiation beam B with sufficient power to supply each ofthe lithographic apparatus LA_(a)-LA_(n). As noted above, the radiationsource SO may comprise a free electron laser.

FIG. 3 is a schematic depiction of a free electron laser FEL comprisingan injector 21, a linear accelerator 22, a bunch compressor 23, anundulator 24, an electron decelerator 26 and a beam dump 100.

The injector 21 is arranged to produce a bunched electron beam E andcomprises an electron source (for example a thermionic cathode or aphoto-cathode) and an accelerating electric field. Electrons in theelectron beam E are further accelerated by the linear accelerator 22. Inan example, the linear accelerator 22 may comprise a plurality of radiofrequency cavities, which are axially spaced along a common axis, andone or more radio frequency power sources, which are operable to controlelectromagnetic fields along the common axis as bunches of electronspass between them so as to accelerate each bunch of electrons. Thecavities may be superconducting radio frequency cavities.Advantageously, this allows: relatively large electromagnetic fields tobe applied at high duty cycles; larger beam apertures, resulting infewer losses due to wakefields; and for the fraction of radio frequencyenergy that is transmitted to the beam (as opposed to dissipated throughthe cavity walls) to be increased. Alternatively, the cavities may beconventionally conducting (i.e. not superconducting), and may be formedfrom, for example, copper. Other types of linear accelerators may beused such as, for example, laser wake-field accelerators or inverse freeelectron laser accelerators.

Optionally, the electron beam E passes through a bunch compressor 23,disposed between the linear accelerator 22 and the undulator 24. Thebunch compressor 23 is configured to spatially compress existing bunchesof electrons in the electron beam E. One type of bunch compressor 23comprises a radiation field directed transverse to the electron beam E.An electron in the electron beam E interacts with the radiation andbunches with other electrons nearby. Another type of bunch compressor 23comprises a magnetic chicane, wherein the length of a path followed byan electron as it passes through the chicane is dependent upon itsenergy. This type of bunch compressor may be used to compress bunches ofelectrons which have been accelerated in a linear accelerator 22 by aplurality of resonant cavities.

The electron beam E then passes through the undulator 24. Generally, theundulator 24 comprises a plurality of modules (not shown). Each modulecomprises a periodic magnet structure, which is operable to produce aperiodic magnetic field and is arranged so as to guide the relativisticelectron beam E produced by the injector 21 and linear accelerator 22along a periodic path within that module. The periodic magnetic fieldproduced by each undulator module causes the electrons to follow anoscillating path about a central axis. As a result, within eachundulator module, the electrons radiate electromagnetic radiationgenerally in the direction of the central axis of that undulator module.

The path followed by the electrons may be sinusoidal and planar, withthe electrons periodically traversing the central axis. Alternatively,the path may be helical, with the electrons rotating about the centralaxis. The type of oscillating path may affect the polarization ofradiation emitted by the free electron laser. For example, a freeelectron laser which causes the electrons to propagate along a helicalpath may emit elliptically polarized radiation.

As electrons move through each undulator module, they interact with theelectric field of the radiation, exchanging energy with the radiation.In general the amount of energy exchanged between the electrons and theradiation will oscillate rapidly unless conditions are close to aresonance condition. Under resonance conditions, the interaction betweenthe electrons and the radiation causes the electrons to bunch togetherinto microbunches, modulated at the wavelength of radiation within theundulator, and coherent emission of radiation along the central axis isstimulated. The resonance condition may be given by:

$\begin{matrix}{{\lambda_{em} = {\frac{\lambda_{u}}{2\;\gamma^{2}}( {1 + \frac{K^{2}}{A}} )}},} & (1)\end{matrix}$where λ_(em) is the wavelength of the radiation, λ_(u) is the undulatorperiod for the undulator module that the electrons are propagatingthrough, γ is the Lorentz factor of the electrons and K is the undulatorparameter. A is dependent upon the geometry of the undulator 24: for ahelical undulator that produces circularly polarized radiation A=1, fora planar undulator A=2, and for a helical undulator which produceselliptically polarized radiation (that is neither circularly polarizednor linearly polarized) 1<A<2. In practice, each bunch of electrons willhave a spread of energies although this spread may be minimized as faras possible (by producing an electron beam E with low emittance). Theundulator parameter K is typically approximately 1 and is given by:

$\begin{matrix}{{K = \frac{q\;\lambda_{u}B_{0}}{2\;\pi\;{mc}}},} & (2)\end{matrix}$where q and m are, respectively, the electric charge and mass of theelectrons, B₀ is the amplitude of the periodic magnetic field, and c isthe speed of light.

The resonant wavelength λ_(em) is equal to the first harmonic wavelengthspontaneously radiated by electrons moving through each undulatormodule. The free electron laser FEL may operate in self-amplifiedspontaneous emission (SASE) mode. Operation in SASE mode may require alow energy spread of the electron bunches in the electron beam E beforeit enters each undulator module. Alternatively, the free electron laserFEL may comprise a seed radiation source, which may be amplified bystimulated emission within the undulator 24. The free electron laser FELmay operate as a recirculating amplifier free electron laser (RAFEL),wherein a portion of the radiation generated by the free electron laserFEL is used to seed further generation of radiation.

Electrons moving through the undulator 24 may cause the amplitude ofradiation to increase, i.e. the free electron laser FEL may have anon-zero gain. Maximum gain may be achieved when the resonance conditionis met or when conditions are close to but slightly off resonance.

An electron which meets the resonance condition as it enters theundulator 24 will lose (or gain) energy as it emits (or absorbs)radiation, so that the resonance condition is no longer satisfied.Therefore, in some embodiments the undulator 24 may be tapered. That is,the amplitude of the periodic magnetic field and/or the undulator periodλ_(u) may vary along the length of the undulator 24 in order to keepbunches of electrons at or close to resonance as they are guided thoughthe undulator 24. The tapering may be achieved by varying the amplitudeof the periodic magnetic field and/or the undulator period λ_(u) withineach undulator module and/or from module to module. Additionally oralternatively tapering may be achieved by varying the helicity of theundulator 24 (by varying the parameter A) within each undulator moduleand/or from module to module.

Radiation produced within the undulator 24 is output as a radiation beamB_(FEL).

After leaving the undulator 24, the electron beam E is absorbed by adump 100. The dump 100 may comprise a sufficient quantity of material toabsorb the electron beam E. The material may have a threshold energy forinduction of radioactivity. Electrons entering the dump 100 with anenergy below the threshold energy may produce only gamma ray showers butwill not induce any significant level of radioactivity. The material mayhave a high threshold energy for induction of radioactivity by electronimpact. For example, the beam dump may comprise aluminium (Al), whichhas a threshold energy of around 17 MeV. It may be desirable to reducethe energy of electrons in the electron beam E before they enter thedump 100. This removes, or at least reduces, the need to remove anddispose of radioactive waste from the dump 100. This is advantageoussince the removal of radioactive waste requires the free electron laserFEL to be shut down periodically and the disposal of radioactive wastecan be costly and can have serious environmental implications.

The energy of electrons in the electron beam E may be reduced beforethey enter the dump 100 by directing the electron beam E through adecelerator 26 disposed between the undulator 24 and the beam dump 100.

In an embodiment the electron beam E which exits the undulator 24 may bedecelerated by passing the electrons back through the linear accelerator22 with a phase difference of 180 degrees relative to the electron beamproduced by the injector 21. The RF fields in the linear acceleratortherefore serve to decelerate the electrons which are output from theundulator 24 and to accelerate electrons output from the injector 21. Asthe electrons decelerate in the linear accelerator 22 some of theirenergy is transferred to the RF fields in the linear accelerator 22.Energy from the decelerating electrons is therefore recovered by thelinear accelerator 22 and may be used to accelerate the electron beam Eoutput from the injector 21. Such an arrangement is known as an energyrecovery linear accelerator (ERL).

In some embodiments of a lithographic system LS the radiation source SOmay comprise a single free electron laser FEL. In such embodiments themain beam B which is emitted from the radiation source SO may be a laserbeam B_(FEL) which is emitted from the free electron laser FEL. In otherembodiments, a lithographic system LS may comprise a plurality of freeelectron lasers. A plurality of laser beams B_(FEL) emitted from thefree electron lasers may be combined to form a single main beam Bcomprising radiation emitted from the plurality of free electron lasersFEL.

FIG. 4 is a schematic illustration of a portion of a lithographic systemLS according to an embodiment of the invention. The portion of thelithographic system LS which is shown in FIG. 4 comprises a plurality ofoptical elements configured to direct a branch radiation beam B_(a) to alithographic apparatus LA_(a). The plurality of optical elementsincludes a first mirror 103, a first variable attenuation mirror 105 a,a second variable attenuation mirror 105 b, a first focusing optic 107,a radiation alteration device 101, a second focusing optic 109 and athird focusing optic 111. The plurality of optical elements which areshown in FIG. 4 may form part of the beam delivery system BDS which isshown schematically in FIG. 1. The beam delivery system BDS may howevercomprise more components than are shown in FIG. 4. For example, the beamdelivery system BDS may further comprise beam splitting optics and mayoptionally also comprise beam expanding optics and/or beam shapingoptics which are not shown in FIG. 4.

The first mirror 103 receives the branch radiation beam B_(a) anddirects the branch radiation beam B_(a) to be incident on the firstattenuation mirror 105 a which subsequently directs the branch radiationbeam B_(a) to be incident on the second attenuation mirror 105 b. Theorientation of the first and second attenuation mirrors 105 a, 105 b areadjustable so as to vary the angle of incidence with which the branchradiation beam B_(a) is incident on the first and second variableattenuation mirrors 105 a, 105 b. The reflectivity of the variableattenuation mirrors 105 a, 105 b is a function of the angle of incidenceof the branch radiation beam B_(a) on the variable attenuation mirrors105 a, 105 b. Varying the angle of incidence with which the branchradiation beam B_(a) is incident on the variable attenuation mirrors 105a, 105 b therefore varies the fraction of the branch radiation beamB_(a) which is reflected at the variable attenuation mirrors. Theorientation of the variable attenuation mirrors 105 a, 105 b maytherefore be controlled in order to control the fraction of the branchradiation beam B_(a) which is reflected at the variable attenuationmirrors, thereby controlling the intensity of the branch radiation beamB_(a) which is provided to the lithographic apparatus LA_(a). Theorientation of the variable attenuation mirrors may be controlled by oneor more actuators (not shown). The one or more actuators may, forexample, be controlled in response to a measurement of the intensity ofthe branch radiation beam B_(a) so as to form a feedback system which isoperable to provide a branch radiation beam B_(a) having a desiredintensity.

The branch radiation beam B_(a) which is reflected from the secondvariable attenuation mirror 105 b is incident on the first focusingoptic 107. The first focusing optic 107 is configured to focus thebranch radiation beam B_(a) to a focal point 108 before the branchradiation beam B_(a) enters the radiation alteration device 101.Focusing the branch radiation beam B_(a) to a focal point 108 before thebranch radiation beam 108 enters the radiation alteration device 101results in the branch radiation beam B_(a) having a positive divergenceas it enters the radiation alteration device 101.

In alternative embodiments the focal point 108 may be positioned insidethe radiation alteration device 101. In such embodiments the branchradiation beam B_(a) may enter the radiation alteration device having anegative divergence. However the divergence of the branch radiation beamB_(a) may become positive after the focal point 108 such that the branchradiation beam B_(a) has a positive divergence in the radiationalteration device.

The radiation alteration device 101 is configured to modify one or moreproperties of the branch radiation beam B_(a) and will be described infurther detail below. The modified branch radiation beam B_(a) which isoutput from the radiation alteration device 101 is incident on thesecond 109 and third 111 focusing optics before the branch radiationbeam B_(a) is provided to the lithographic apparatus LA_(a). The secondand third focusing optics 109, 111 are configured to focus the branchradiation beam B_(a) to an intermediate focus IF. The intermediate focusIF is situated at an opening 8 in an enclosing structure of thelithographic apparatus LA_(a). As was described above with reference toFIG. 2 the lithographic apparatus LA_(a) comprises an illuminationsystem IL, a support structure MT configured to support a patterningdevice MA (e.g. a mask), a projection system PS and a substrate table WTconfigured to support a substrate W.

As was described above the branch radiation beam B_(a) comprises atleast a portion of the EUV radiation which is emitted from a freeelectron laser FEL which forms part of the radiation source SO. Aradiation beam which is output from a free electron laser FEL istypically a coherent, well collimated radiation beam having a relativelysmall etendue. In some embodiments the etendue of a radiation beam whichis emitted from a free electron laser FEL may be sufficiently small thatthe radiation beam is considered to be diffraction limited.

The etendue of a radiation beam in free space (i.e. a medium with arefractive index of 1) at an infinitesimal surface element dS in anoptical system is given by the product of the area of the surface dS,the solid angle dΩ subtended by radiation crossing (or emitted by) thesurface element and the cosine of the angle between the normal to thesurface element and the direction of the radiation crossing that point.In general, the etendue of a radiation beam at an extended surface S isgiven by integrating over the solid angle subtended by radiationcrossing (or emitted by) each surface element (to account for the factthat light may cross each point on the surface at a range of angles) andintegrating over the surface (to sum the contributions from all suchsurface elements). For a light source operable to produce a wellcollimated radiation beam, as is produced by a free electron laser FEL,the etendue of the light source may be estimated by the product of thearea of the light source and the solid angle into which light isemitted. Further, for such a light source the solid angle into whichlight is emitted is given by (using small angle approximations) πθ²,where θ is the half divergence of the light source. Therefore theetendue of such a light source is given by G=πAθ², where A is the areaof the light source. A radiation beam which is emitted from a freeelectron laser FEL may, for example, have a divergence which is lessthan about 500 μrad (in some embodiments the divergence may be less thanabout 100 μrad) and may have a diameter of around 50 μm to 100 μm at itsbeam waist, as it leaves the undulator 24. In an embodiment in which thebeam waist diameter is 50 μm and the beam divergence is 100 μrad theetendue of the radiation beam is around 1.5×10⁻¹¹ mm².

In some embodiments a free electron laser FEL may emit a radiation beamwhich has a Gaussian-like intensity profile. The etendue of a radiationbeam having a Gaussian intensity profile is approximately equal to thewavelength of the radiation beam squared. In some embodiments a freeelectron laser FEL may emit an EUV radiation beam having a wavelength ofapproximately 13.5 nm and having a Gaussian intensity profile. In suchan embodiment the etendue of the radiation beam is approximately1.8×10⁻¹⁶ m². In practice the intensity profile of a radiation beamwhich is emitted from a free electron laser FEL may not be perfectlyGaussian. Consequently the etendue of a radiation beam which is emittedfrom a free electron laser FEL may in practice be approximately 2 or 3times greater than the square of the wavelength of the radiation beam.

The etendue of a radiation beam cannot decrease as it propagates throughan optical system. The etendue of a radiation beam may remain constantas it propagates through an optical system in free space and undergoesreflections and refractions. However, as a radiation beam propagatesthrough an optical system which spreads out radiation, for example byscattering and/or diffraction, its etendue will increase. The higher thequality of the optical elements (for example mirrors and lenses) in theoptical system, the smaller the increase in etendue will be.

The optical elements which form the optical path of a branch radiationbeam B_(a) from a free electron laser FEL to a lithographic apparatusLA_(a) are typically of a high quality such that they result in onlyrelatively small increases in the etendue. If the branch radiation beamB_(a) only passes via optical elements which do not significantlyincrease the etendue of the branch radiation beam B_(a) then the etendueof the branch radiation beam B_(a) which is focused at the intermediatefocus IF will be relatively small and the branch radiation beam B_(a)will be focused to a small point at the intermediate focus IF. As wasdescribed above with reference to FIG. 2, the branch radiation beamB_(a) which is focused at the intermediate focus IF enters theillumination system IL of the lithographic apparatus LA_(a) and isincident on a field facet mirror 10 and a pupil facet mirror 11. Thefield facet mirror 10 and the pupil facet mirror 11 each comprise aplurality of reflective facets which each reflect a portion of thebranch radiation beam B_(a). The portions of the branch radiation beamB_(a) which are reflected at the field facets and pupil facets may bereferred to as sub-beams.

The field facets which form the field facet mirror 10 may focus thesub-beams which are received by the field facets onto the pupil facetswhich form the pupil facet mirror 11. The spot size of each sub-beamwhich is incident on a pupil facet of the pupil facet mirror 11 dependsin part on the etendue of the branch radiation beam B_(a). A branchradiation beam B_(a) having a small etendue may cause the spot sizes ofthe sub-beams of the branch radiation beam B_(a) which are incident onthe pupil facets to be relatively small. A relatively small spot size ona pupil facet causes the sub-beam to be incident on a pupil facet with arelatively high irradiance. A high irradiance on a pupil facet maydamage the pupil facet.

It may therefore be desirable to increase the spot sizes of thesub-beams which are incident on the pupil facets so as to decrease theirradiance on the pupil facets and decrease the likelihood of damagebeing caused to the pupil facets. As was described above the spot sizesof the sub-beams which are incident on the pupil facets may be increasedby increasing the etendue of the branch radiation beam B_(a). As will bedescribed in further detail below the radiation alteration device 101may be configured to increase the etendue of the branch radiation beamB_(a) thereby causing the spot sizes of the sub-beams on the pupilfacets to be increased.

As was mentioned above, a radiation beam which is emitted from a freeelectron laser FEL is typically a coherent radiation beam. When aspatially coherent radiation beam is reflected, for example, at one ormore of the plurality of optical elements shown in FIG. 4, then smallpath length differences may be introduced between different portions ofthe radiation beam, thereby introducing phase differences betweendifferent portions of the radiation beam. Phase differences betweendifferent portions of the radiation beam may cause different portions ofthe radiation beam to interfere with each, thereby forming interferencepatterns. For example, interference between different portions of aradiation beam may lead to the occurrence of a so called specklepattern. In a lithographic apparatus a radiation beam exhibiting aspeckle pattern may disadvantageously cause different portions of asubstrate W to be exposed to different doses of radiation. It maytherefore be desirable to reduce the spatial coherence of the branchradiation beam B_(a) so as to reduce any disadvantageous effects whichresult from interference between different portions of the branchradiation beam B_(a). As will be described in further detail below theradiation alteration device 101 may be configured to reduce the spatialcoherence of the branch radiation beam B_(a) so as to prevent theoccurrence of a speckle pattern in the branch radiation beam B_(a).

FIG. 5 is a schematic illustration of an embodiment of the radiationalteration device 101. The radiation alteration device 101 comprises atube 125 having a first opening 121 and a second opening 122. The firstopening 121 is arranged to receive the branch radiation beam B_(a) andthe second opening 122 is arranged to output the branch radiation beamB_(a). The tube 125 is defined by an internal surface 123 of theradiation alteration device 101. The internal surface 123 issubstantially reflective to EUV radiation and is arranged to cause thebranch radiation beam B_(a) which enters the tube 125 through the firstopening 121 to undergo multiple successive reflections at the internalsurface 123. The multiple successive reflections serve to modify thebranch radiation beam B_(a) before the branch radiation beam B_(a) exitsthe tube 125 through the second opening 122.

The path of the branch radiation beam B_(a) through the radiationalteration device 101 is represented in FIG. 5 with a series of rays 127which are shown propagating through the radiation alteration device 101.Since the branch radiation beam B_(a) is focused to a focal point 108prior to entering the radiation alteration device 101, the rays 127which form the branch radiation beam B_(a) are diverging from each otheras the branch radiation beam B_(a) enters the radiation alterationdevice 101. That is, the branch radiation beam B_(a) has a positivedivergence as it enters the radiation alteration device 101. Thepositive divergence of the branch radiation beam B_(a) means that therays 127 of the branch radiation beam B_(a) are incident on the internalsurface 123 of the radiation alteration device at different positionsand are subsequently reflected along different paths. The rays 127 whichform the branch radiation beam B_(a) therefore propagate alongsubstantially different paths through the radiation alteration device101 such that the rays 127 are spatially scrambled by the radiationalteration device 101.

In the embodiment which is shown in FIG. 5 the tube 125 includes a bend124. The bend 124 has a bend angle α. The bend angle α is measuredrelative to an axis 133 which extends along the cross-sectional centreof the tube 125. The bend angle α is the angle by which the axis 133deviates in the bend 124. In the embodiment which is shown in FIG. 5 thedirection in which the axis 133 extends undergoes a step change at thebend 124 of the tube 125. In other embodiments a tube 125 may include acontinuous bend in which the direction in which the axis 133 extendsundergoes a continuous transition in the bend such that the tube 125includes a curve. Some embodiments of a radiation alteration device mayinclude more than one bend. Other embodiments of a radiation alterationdevice may not include any bends.

As was described above the radiation alteration device 101 causes rays127 of the branch radiation beam B_(a) to be spatially scrambled. Theradiation alteration device 101 may be considered to provide a mappingof the rays 127 which form the branch radiation beam between the firstopening 121 of a radiation alteration device 101 and the second opening122 of the radiation alteration device.

FIG. 6 is a schematic illustration of the cross-sectional position of asubset 127′ of the rays 127 which form a branch radiation beam B_(a) atthe first opening 121 of the radiation alteration device 101. In orderto illustrate the spatial scrambling of the rays 127 which is caused bythe radiation alteration device only a subset 127′ of rays is shown inFIG. 6. The subset 127′ of rays represents a half segment of the rays127 which form the branch radiation beam B_(a). In practice the branchradiation beam B_(a) may extend over approximately twice the area whichis indicated by the rays 127 which are shown in FIG. 6. The full extentof the cross-section of the branch radiation beam at the first opening121 of the radiation alteration device 101 is indicated in FIG. 6 with adashed circle 131 a.

FIG. 7 is a schematic illustration of the cross-sectional position ofthe subset 127′ of rays of FIG. 6 at the second opening 122 of theradiation alteration device 101. The full extent of the cross-section ofthe branch radiation beam B_(a) at the second opening 122 of theradiation alteration device 101 is indicated in FIG. 7 with a dashedcircle 131 b. FIG. 7 is representative of the mapping of rays betweenthe first opening 121 and the second opening 122. It can be seen fromFIG. 7 that the subset 127′ of rays which form a half segment of thebranch radiation beam B_(a) at the first opening 121 are redistributedacross the full extent of the cross-section of the branch radiation beamB_(a) at the second opening 122. That is, the cross-sectional positionof the subset 127′ of rays is spatially scrambled by the radiationalteration device 101.

The mapping of rays 127 between the first opening 121 and the secondopening 122 which is shown in FIGS. 6 and 7 is presented merely as anillustrative example of a mapping of rays which may be caused by aradiation alteration device. The example mapping which is illustrated inFIGS. 6 and 7 may not, for example, correspond exactly with the mappingwhich results from the embodiment of a radiation alteration device 101which is shown in FIG. 5. Different embodiments of a radiationalteration device may result in different mappings and different degreesof spatial scrambling.

Spatially scrambling the rays which form the branch radiation beam B_(a)advantageously increases the spatial homogeneity of the spatialintensity profile of the branch radiation beam B_(a). That is, thespatial intensity profile of the branch radiation beam B_(a) which exitsthe radiation alteration device 101 through the second opening 122 ofthe radiation alteration device 101 is more homogenous than the spatialintensity profile of the branch radiation B_(a) which enters theradiation alteration device 101 through the first opening 121.

In general, the intensity of a branch radiation beam B_(a) may vary atdifferent positions in a cross-section through the branch radiation beamB_(a). For example, the branch radiation beam B_(a) may have asubstantially Gaussian-like spatial intensity profile. The spatialintensity profile of the branch radiation beam before the radiationalteration device 101 may therefore, in general, be considered to benon-homogenous. As was described above the branch radiation beam B_(a)enters the lithographic apparatus LA_(a) and is incident on the fieldfacet mirror 10 and the pupil facet mirror 11 in the illumination systemIL.

The field facet mirror 10 and the pupil facet mirror 11 together serveto condition the branch radiation beam B_(a) before it is incident upona patterning device MA. In particular, the field facet mirror 10 and thepupil facet mirror 11 may be configured to provide a radiation beamhaving a desired angular and spatial intensity profile. In order toachieve this the field facets which form the field facet mirror 10 andthe pupil facets which form the pupil facet mirror 11 are orientated soas to direct different portions of the intensity profile of the branchradiation beam B_(a) in different directions, so as to form a radiationbeam having a desired angular and spatial intensity profile. Theorientation of the field facets and pupil facets is based upon receivinga branch radiation beam B_(a) having a known spatial intensity profileat the field facet mirror 10. For example, each facet may be orientatedaccording to a known intensity of radiation which will be incident onthat facet.

If the branch radiation beam B_(a) which is provided to the lithographicapparatus LA_(a) has a non-homogenous spatial intensity profile at thefield facet mirror 10 (e.g. if no radiation alteration device 101 isprovided before the lithographic apparatus LA_(a)) then the intensity ofradiation which is incident on each field facet of the field facetmirror 11 is sensitive to changes in the pointing direction of thebranch radiation beam B_(a). For example, a change in the pointingdirection of the branch radiation beam B_(a) will result in a change inthe portion of the intensity profile which is incident on each fieldfacet. If the branch radiation beam B_(a) which is incident on the fieldfacet mirror 10 has a non-homogenous spatial intensity profile then achange in the portion of the intensity profile which is incident on eachfield facet will result in a change in the intensity of radiation whichis incident on each field facet. A change in the intensity of radiationwhich is incident on each field facet will further result in a change inthe angular and spatial intensity profile of the radiation beam which isincident on a patterning device MA. In particular, the angular andspatial intensity profile of the radiation beam which is incident on apatterning device MA may deviate from a desired angular and spatialintensity profile.

In the embodiment which is shown in FIG. 4 the branch radiation beamB_(a) which is provided to the lithographic apparatus LA_(a) has passedthrough the radiation alteration device 101. As was described above, byspatially scrambling the rays 127 which form the branch radiation beamB_(a), the radiation alteration device serves to increase thehomogeneity of the spatial intensity profile of the branch radiationbeam B_(a). Consequently, the branch radiation beam B_(a) which isprovided to the lithographic apparatus and which is incident on thefield facet mirror 10 may have a relatively homogenous spatial intensityprofile.

In embodiments in which the branch radiation beam B_(a) which isincident on the field facet mirror 10 has a relatively homogenousspatial intensity profile, the different portions of the intensityprofile which are incident on each of the field facets of the fieldfacet mirror 10 have similar intensities. Accordingly a change in thepointing direction of the branch radiation beam B_(a) (which causes achange in the portion of the cross-section of the radiation beam whichis incident on each field facet) will result in only small changes inthe intensity of radiation which is incident on each field facet.Consequently the angular and spatial intensity profile of the radiationbeam which is incident on a patterning device MA is less sensitive tothe pointing direction of the branch radiation beam B_(a) in embodimentsin which the spatial intensity profile of the branch radiation beamB_(a) which is incident on the field facet mirror 10 is relativelyhomogenous (when compared to embodiments in which the spatial intensityprofile is relatively in homogenous).

The pointing direction of the branch radiation beam B_(a) which isprovided to the lithographic apparatus LA_(a) is a function of thepointing direction of a radiation beam which is emitted by a freeelectron laser FEL (which forms part of the radiation source SO), andthe position and orientation of the optical elements at which the branchradiation beam B_(a) is reflected on its optical path to thelithographic apparatus LA_(a). Both the pointing direction of aradiation beam which is emitted from a free electron laser FEL and theorientation of optical elements at which the branch radiation beam B_(a)is reflected may vary over time. For example, the pointing direction ofa radiation beam which is emitted by a free electron laser FEL dependson the periodic path through the undulator 23 along which the electronbeam E is guided by a periodic magnetic field in the undulator 23. Theperiodic magnetic field in the undulator 23 may change over time (e.g.due to changes in the magnetism of one or more magnets which generatethe periodic magnetic field), thereby causing a change in the path ofthe electron beam E through the undulator 23 and a corresponding changein the pointing direction of the radiation beam emitted by the freeelectron laser FEL. Additionally or alternatively the position and/ororientation of the optical elements at which the branch radiation beamB_(a) is reflected on its optical path to the lithographic apparatusLA_(a) may vary over time, thereby causing a change in the beam pointingof the branch radiation beam B_(a). It is therefore desirable to reducethe sensitivity of the intensity profile of the radiation beam which isincident on a patterning device MA in the lithographic apparatus LA_(a)to variations in the pointing direction of the branch radiation beamB_(a).

As was described above, providing a radiation alteration device 101 inthe optical path of the branch radiation beam B_(a) advantageouslyincreases the homogeneity of the spatial intensity profile of the branchradiation beam B_(a) which is incident on the field facet mirror.Providing a radiation alteration device 101 therefore reduces thesensitivity of the spatial intensity profile of the branch radiationbeam B_(a) which is incident on the field facet mirror 10, to changes inthe pointing direction of the branch radiation beam B_(a). Accordinglythe sensitivity of the spatial and angular intensity profile of theradiation beam which is incident on a patterning device MA isadvantageously reduced.

In order to sufficiently increase the homogeneity of a branch radiationbeam B_(a) it may be desirable to configure a radiation alterationdevice such that the quantity θL/D is greater than or equal to about 10,where L is the length of the radiation alteration device (i.e. thelength between the first and second openings), θ is the half-divergence(in radians) of the branch radiation beam B_(a) which enters theradiation alteration device and D is the diameter of the second openingof the radiation alteration device. Configuring a radiation alterationdevice such that the quantity θL/D is greater than or equal to about 10may ensure that the rays 127 which form the branch radiation beam B_(a)undergo a sufficient number of reflections at the internal surface ofthe radiation alteration device so as to bring about a desirableincrease in the homogeneity of the branch radiation beam B_(a).

The spatial intensity profile of a modified branch radiation beam B_(a)which is output from the radiation alteration device may be relativelyhomogenous at the second opening 122 of the radiation alteration devicebut may be less homogenous at some locations further downstream of theradiation alteration (a downstream direction corresponds with thegeneral direction of propagation of the branch radiation beam B_(a) andan upstream direction corresponds with a reverse of the downstreamdirection). In order to increase the homogeneity of the radiation beamwhich is incident on the field facet mirror 10, the second and thirdfocusing optics 109, 111 may be configured to form an image of thesecond opening 122 of the radiation alteration device 101 at the fieldfacet mirror 10. Embodiments of focusing schemes which may be used toform an image at the field facet mirror 10 are described further belowwith reference to FIGS. 12A and 12B.

As was described above, different rays 127 which are input to theradiation alteration device 101 through the first opening 121 of theradiation alteration device 101 propagate along different paths throughthe radiation alteration device 101. The different paths through theradiation alteration device 101 may have different lengths andconsequently path length differences are introduced between thedifferent rays 127 which are output from the second opening 122 of theradiation alteration device 101. As was described above the branchradiation beam B_(a) which enters the radiation alteration device may bea coherent radiation beam. The introduction of path length differencesbetween different rays 127 of the branch radiation beam B_(a) may causea reduction in the spatial coherence of the branch radiation beam B_(a).Reducing the spatial coherence of the branch radiation beam B_(a) mayadvantageously reduce the likelihood of interference between differentportions of the branch radiation beam B_(a) resulting in the formationof an interference pattern such as a speckle pattern. The radiationalteration device 101 may therefore advantageously reduce the likelihoodof a speckle pattern forming in a lithographic apparatus LA_(a) (whencompared to a lithographic system in which no radiation alterationdevice is provided).

The spatial coherence of the branch radiation beam B_(a) may be reducedto an extent which prevents the formation of interference patterns (e.g.a speckle pattern) if the range of different path lengths along whichdifferent rays 127 of the branch radiation beam B_(a) propagate, isgreater than the coherence length of the branch radiation beam B_(a).The coherence length of the branch radiation beam B_(a) (which comprisesEUV radiation) may be of the order of about 1 μm. The range of differentpath lengths along which different rays 127 of the branch radiation beamB_(a) may be of the order of approximately Lθ² where L is the length ofthe radiation alteration device and θ is the half-divergence (inradians) of the branch radiation beam B_(a) which enters the radiationalteration device. It may therefore be desirable to configure aradiation alteration device and/or one or more focussing elements whichdetermine the divergence of the branch radiation beam B_(a), such thatLθ² is much greater than the coherence length of the branch radiationbeam B_(a).

The different paths along which different rays 127 of the branchradiation beam propagate through the radiation alteration device 101 mayadditionally serve to scramble the polarization state of the branchradiation beam B_(a). In general it is desirable to expose a substrate Win a lithographic apparatus to radiation which does not have apreferential polarization direction. For example, a substrate W may beexposed to radiation which is circularly polarized or which isunpolarized. However it may be undesirable to expose a substrate W toradiation which is linearly polarized or which is elliptically polarizedsince a linear or elliptical polarization state has a preferentialpolarization direction.

A radiation beam which is emitted from a free electron laser FEL istypically a polarized radiation beam. The radiation beam which isemitted from a free electron laser FEL may be linearly, elliptically orcircularly polarized depending on the configuration of the undulator 23from which the radiation beam is emitted. For example, a free electronlaser FEL having an undulator which causes electrons to propagate alonga helical path may emit circularly polarized radiation and a freeelectron laser FEL having an undulator which causes electrons topropagate along a planar path may emit linearly polarized radiation.

As has been described above, the optical path of a branch radiation beamB_(a) from a free electron laser FEL to a lithographic apparatus LA_(a)may include multiple reflections at a variety of different opticalelements. One or more of the reflections which a branch radiation beamB_(a) undergoes may alter the polarization state of the branch radiationbeam B_(a). For example, reflection of the branch radiation beam B_(a)may cause a phase retardance between orthogonally polarized componentsof the branch radiation beam B_(a). Introducing a phase retardance to acircularly polarized radiation beam or a linearly polarized radiationbeam typically converts the circular or linear polarization state to anelliptically polarized state. Consequently a branch radiation beam B_(a)which is emitted from a free electron laser FEL typically becomeselliptically polarized as it propagates along its optical path to alithographic apparatus LA_(a). In the absence of a radiation alterationdevice 101, a branch radiation beam B_(a) may therefore be undesirablyprovided to a lithographic apparatus LA_(a) with an ellipticalpolarization state which has a preferential polarization direction.

As has been described above the radiation alteration device 101 causesdifferent rays 127 to undergo different paths through the radiationalteration device 101 and to undergo reflections at different grazingangles in the radiation alteration device 101. Consequently the rays 127which are output from the radiation alteration device 101 have differentpolarization states. Taken as a whole the modified branch radiation beamB_(a) which is output from the radiation alteration device 101 does nottherefore have a preferential polarization direction. Advantageouslythis results in the exposure of a substrate W in a lithographicapparatus LA_(a) to radiation which does not have a preferentialpolarization direction.

In addition to the spatial scrambling of rays 127 which form the branchradiation beam B_(a) which is caused by the radiation alteration device101, the radiation alteration device also serves to significantlyincrease the etendue of the branch radiation beam B_(a). That is, theetendue of the branch radiation beam B_(a) which exits the radiationalteration device 101 through the second opening 122 is greater than theetendue of the branch radiation beam B_(a) which enters the radiationalteration device 101 through the first opening 121.

In order to aid understanding of the increase in the etendue of thebranch radiation beam B_(a), which is caused by a radiation alterationdevice 101, a description is given below of the effect of a simpleradiation alteration device. FIG. 8A is a schematic illustration of anembodiment of a radiation alteration device 1011 shown in perspectiveview. Also shown in FIG. 8A is a cartesian coordinate system which isused consistently throughout the Figures. The radiation alterationdevice 1011 comprises a tube 1251 having a first opening 1211 arrangedto receive a branch radiation beam B_(a) and a second opening 1221arranged to output the radiation beam B_(a) after modification of thebranch radiation beam B_(a) by the radiation alteration device 1011. Thetube 1251 is defined by an internal surface 1231 which is substantiallyreflective to EUV radiation. In the example which is shown in FIG. 8Athe cross-sectional shape of the internal surface 1231 of the tube 1251is a square. The radiation alteration device 1011 has an optical axis1331 which extends through the cross-sectional centre of the tube 1251and in the z-direction indicated in FIG. 8A. For ease of illustrationand explanation, the embodiment of the radiation alteration device 1011which is shown in FIG. 8A does not include a bend. However otherembodiments of a radiation alteration device may include a bend (as isshown, for example, in FIG. 5).

The radiation alteration device 1011 receives a branch radiation beamB_(a) which is represented in FIG. 8A as a series of rays 127 whichextend from a focal point 1081. The branch radiation beam B_(a) may befocused to the focal point 1081 prior to the branch radiation beam B_(a)entering a radiation alteration device by one or more focusing optics(as is shown, for example, in FIG. 4). The focal point 1081 is situatedupstream of the first opening 1211 and on the optical axis 1331. Therays 127 which are shown in FIG. 8A are therefore diverging from eachother as they enter the radiation alteration device 1211, each rayforming an angle with the optical axis 1331. Whilst not shown in FIG. 8Athe rays 127 enter the tube 1251 through the first opening 1211 andundergo multiple successive reflections at the internal surface 1231. Inthe example which is shown in FIG. 8A in which the cross-sectional shapeof the internal surface 1231 of the tube 1251 is a square, the anglewhich each ray forms with the optical axis 1331 is conserved as the rayspropagate along the tube 1251 and undergoes reflections at the internalsurface 1231.

The result of the multiple reflections of the rays 127 at the internalsurface 1231 will now be described with reference to FIG. 8B in whichthe radiation alteration device 1011 is shown in the y-z plane.Radiation which is output from the second opening 1221 of the radiationalteration device 1251 and which is incident on a plane 1291 situateddownstream of the radiation alteration device 1011 is equivalent toradiation being emitted from a plurality of virtual point sources 1351which each illuminate the entire second opening 1221 of the radiationalteration device 1011. By way of example, five virtual point sources1351 are shown in FIG. 8B, each of which illuminate the full extent ofthe second opening 1221 of the radiation alteration device 1011. Theillumination of the second opening 1221 by the virtual point sources1351 is indicated in FIG. 8B with a series of dashed, dotted anddash-dot lines which indicate the outer extent of radiation emanatingfrom each virtual point source 1351. In practice the radiation which isoutput from the second opening of the radiation alteration device 1011and which is incident on a plane 1291 situated downstream of theradiation alteration device 1011 may be equivalent to radiation emittedfrom many more than five virtual point sources 1351, however for ease ofillustration only five virtual point sources 1351 are shown in FIG. 8B.

It can be seen from FIG. 8B that the radiation emanating from thedifferent point sources 1351 overlaps with each other so as to form amodified branch radiation beam B_(a) downstream of the radiationalteration device (e.g. in the plane 1291 shown in FIG. 8B) which has anapparent source size of the area of the second opening 1221 of theradiation alteration device 1011.

As was described above the etendue G of a radiation beam may beestimated by G=πAθ² where θ is the half divergence of the radiation beamand A is the area of the light source from which radiation is emitted(or equivalently the apparent source size of the radiation beam). Theradiation beam which enters the radiation alteration device 1011 throughthe first opening 1211 radiates from a diffraction limited focal point1081 with a given divergence. The apparent source size A of the branchradiation beam B_(a) which enters the radiation alteration device 1011is therefore equivalent to the size of the focal point 1081. As wasexplained above the modified branch radiation beam B_(a) which is outputfrom the second opening 1221 of the radiation alteration device 1011 hasan apparent source size A corresponding to the area of the secondopening 1221 of the radiation alteration device 1221. The area of thesecond opening 1221 is significantly larger than the size of thediffraction limited focal point 1081 and thus the radiation alterationdevice 1221 significantly increases the apparent source size of thebranch radiation beam B_(a).

As was explained above, the angle which each ray 127, which enters theradiation alteration device 1011, forms with the optical axis 1331 isconserved during reflections of the rays 127 at the internal surface1231 of the radiation alteration device 1011. The conservation of theangle which each ray 127 forms with the optical axis 1331 means that themaximum angle (relative to the optical axis) with which a ray 127 isoutput from the second opening 1221 of the radiation alteration device1011 is the same as the maximum angle with which a ray 127 enters theradiation alteration device 1011. Consequently the divergence of themodified branch radiation beam B_(a) which is output from the secondopening 1221 is approximately the same as the divergence of the branchradiation beam B_(a) which enters the radiation alteration device 1011through the first opening 1211.

The effect of the radiation alteration device 1221 on the branchradiation beam B_(a) is therefore to increase the apparent source sizeof the branch radiation beam B_(a) whilst causing little or no change tothe divergence of the branch radiation beam B_(a). Since the etendue ofthe branch radiation beam B_(a) is approximately proportional to theproduct of the square of the half divergence θ of the branch radiationbeam B_(a) and the apparent source size A of the branch radiation beamB_(a), the radiation alteration device 1221 advantageously causes asignificant increase in the etendue of the branch radiation beam B_(a).

In some embodiments the etendue of the modified branch radiation beamB_(a) which exits the radiation alteration device 101 may be greaterthan at least 10 times the etendue of the branch radiation beam B_(a)which enters the radiation alteration device 101. In some embodimentsthe etendue of the modified branch radiation beam B_(a) which exits theradiation alteration device 101 may be several orders of magnitudegreater than the etendue of the branch radiation beam B_(a) which entersthe radiation alteration device 101. For example, in some embodimentsthe etendue of the modified branch radiation beam B_(a) which exits theradiation alteration device may be at least 1×10⁴, 1×10⁶ or 1×10⁸ timesthe etendue of the branch radiation beam B_(a) which enters theradiation alteration device 101. In some embodiments the radiationalteration device 101 may be configured to increase the etendue of thebranch radiation beam B_(a) such that the etendue of the branchradiation beam B_(a) which is output from the radiation alterationdevice has an etendue which is greater than about 1×10⁻⁸ m². Forexample, the radiation alteration device 101 may be configured toincrease the etendue of the branch radiation beam B_(a) such that theetendue of the branch radiation beam B_(a) which is output from theradiation alteration device 101 is approximately 3×10⁻⁸ or 5×10⁻⁸ m².

The etendue of the modified branch radiation beam B_(a) which is outputfrom a radiation alteration device may constrain the minimum spot sizeto which the modified radiation beam B_(a) can be focused at, forexample, an intermediate focus IF. The spot size to which a modifiedradiation beam B_(a) is focused at an intermediate focus IF depends onthe etendue of the modified branch radiation beam B_(a) and the focallength of a focusing optic which focuses the modified radiation beamB_(a). In some embodiments, it may be desirable to provide alithographic apparatus with a modified branch radiation beam B_(a)having a desired divergence. For example, it may be desirable to providea lithographic apparatus with a modified branch radiation beam B_(a)having a half divergence θ which is about 0.2 radians. The focal lengthof a focusing optic which focuses the branch radiation beam B_(a) to anintermediate focus IF may be configured such that the divergence of thebranch radiation beam B_(a) which enters a lithographic apparatus LA_(a)(having passed through the intermediate focus IF) is close to thedesired divergence. It may be desirable to limit the etendue of amodified branch radiation beam B_(a) such that when the modified branchradiation beam B_(a) is focused to the intermediate focus IF, the spotsize of the modified branch radiation beam B_(a) at the intermediatefocus IF is no larger than a desired spot size at the intermediate focusIF.

Whilst an increase in the etendue of the branch radiation beam B_(a) hasbeen described above with reference to a specific embodiment of aradiation alteration device 1011, having a square cross-section and notincluding a bend, other embodiments of a radiation alteration which areconfigured differently to the embodiment shown in FIG. 8A (e.g. having adifferent cross-sectional shape and/or including one or more bends) mayalso cause an increase in the etendue of the branch radiation beam B_(a)in a similar manner as has described above with reference to FIGS. 8Aand 8B.

As was described above with reference to FIG. 8B the radiation which isoutput from the second opening 1221 of the radiation alteration device1011 is equivalent to radiation emanating from a plurality of virtualpoint sources 1351 which each illuminate the entire second opening 1221of the radiation alteration device 1011. FIG. 8C is a schematicillustration of the virtual point sources 1351 as viewed from a plane1291 which lies downstream of the radiation alteration device 1011 andwhich is perpendicular to the z-direction. It can be seen from FIG. 8Cthat the radiation which is output from the second opening 1221 isequivalent to radiation emanating from a square shaped uniform grid ofvirtual point sources 1351 which extend around the radiation alterationdevice 1011 in the x and y-directions.

As was described above each virtual point source 1351 illuminates theentire second opening 1221 of the radiation alteration device 1011. Inorder to demonstrate the shape and extent of the radiation which isoutput from the radiation alteration device 1011, the radiationemanating from four virtual point sources 1351 a, 1351 b, 1351 c and1351 d which form the corners of the grid of virtual point sources 1351is shown with lines 1371 a, 1371 b, 1371 c and 1371 d in FIG. 8C. Thecross-sectional extent of radiation emanating from each of the cornervirtual point sources 1351 a, 1351 b, 1351 c and 1351 d in the plane1291 is depicted in FIG. 8C with dashed boxes 1391 a, 1391 b, 1391 c and1391 d. The radiation which emanates from the corner virtual pointsources 1351 a, 1351 b, 1351 c and 1351 d forms the corners of theextent of the modified branch radiation beam B_(a) in the plane 1291downstream of the second opening 1221 of the radiation alteration device1011. The extent of the modified branch radiation beam B_(a) in theplane 1291 is indicated in FIG. 8C with a dashed box 1399. It can beseen from FIG. 8C that the modified branch radiation beam B_(a) in theplane 1291 has a cross-sectional shape which is a square.

In general the cross-sectional shape of a modified branch radiation beamB_(a) which is output from a radiation alteration device corresponds tothe cross-sectional shape of the radiation alteration device. In otherembodiments a radiation alteration device may have a cross-sectionalshape other than a square. For example, some embodiments of a radiationalteration device may have an elliptical or circular cross-sectionalshape (as is shown, for example, in FIG. 5). In other embodiments aradiation alteration device may have a cross-sectional shape which is apolygon. For example, the cross-sectional shape of a radiationalteration device may be a triangle, a square, a pentagon, a hexagon oranother polygonal shape. In some embodiments the cross-sectional shapeof a radiation alteration device may vary along its length. For example,the cross-sectional area of the radiation alteration device may increasealong its length.

The cross-sectional shape of a radiation alteration device determinesboth the distribution of virtual source points 1351 and thecross-sectional shape of a modified branch-radiation beam B_(a) which isoutput from the radiation alteration device 1011. The cross-sectionalshape of a radiation alteration device may therefore be selected inorder to provide a modified branch radiation beam B_(a) having a desiredcross-sectional shape. For example, the cross-sectional shape of aradiation alteration device may be selected so as to match thecross-sectional shape of the field facet mirror 10 on which the modifiedbranch radiation beam B_(a) is incident after having passed through anintermediate focus IF.

In some embodiments the field facet mirror 10 on which a modified branchradiation beam B_(a) is incident has a generally circular shape. In suchembodiments a radiation alteration device may be used which has agenerally circular cross-sectional shape such that the shape of themodified branch radiation beam B_(a) corresponds with the shape of thefield facet mirror 10. Providing a modified branch radiation beam B_(a)which has a cross-sectional shape corresponding with the cross-sectionalshape of the field facet mirror 10 allows all of the field facets whichform the field facet mirror 10 to be illuminated with the branchradiation beam B_(a) whilst reducing any extent of the branch radiationbeam B_(a) which is not incident on a field facet and is therefore notdirected to the pupil facet mirror 11 and is thus lost from the branchradiation beam B_(a).

However, in some applications it may be desirable to use a radiationalteration device having a cross-sectional shape which is a polygon. Aradiation alteration device have a polygonal cross-sectional shape mayoutput a modified branch radiation beam B_(a) whose angular and spatialintensity profile which is more homogenous than the angular and spatialintensity profile of a modified branch radiation B_(a) which is outputfrom a radiation alteration device having a circular or ellipticalcross-sectional shape (assuming the same input branch radiation beamB_(a)). In embodiments in which it is desired to provide a branchradiation beam B_(a) having a homogenous angular and spatial intensityprofile it may therefore be desirable to use a radiation alterationdevice having a polygonal cross-sectional shape. In such embodiments apolygonal shape which is relatively similar to the cross-sectional shapeof the field facet mirror 10 may be used so as to reduce the radiationwhich is lost from the branch radiation beam B_(a) due to differencesbetween the cross-sectional shape of the branch radiation beam B_(a) andthe cross-sectional shape of the field facet mirror 10. For example, inan embodiment in which the field facet mirror 10 has a generallycircular shape, a radiation alteration device having a hexagonalcross-section may be used. The use of a radiation alteration devicehaving a hexagonal cross-section may reduce the amount of radiationwhich is lost from the radiation beam B_(a) due to differences betweenthe cross-sectional shape of the branch radiation beam B_(a) and thecross-sectional shape of the field facet mirror 10 than when comparedto, for example, the use of a radiation alteration device having asquare shaped cross-section.

In other embodiments the field facet mirror 10 may have a shape otherthan a generally circular shape. For example, in some embodiments thefield facet mirror 10 may have a generally rectangular shape. In suchembodiments a radiation alteration device having a rectangularcross-section may be used in order to provide a branch radiation beamB_(a) having a cross-sectional shape which is similar to thecross-sectional shape of the field facet mirror 10.

In addition to the influence of the cross-sectional shape of a radiationalteration device on the cross-sectional shape of the modified branchradiation beam B_(a) which is output from the radiation alterationdevice, the shape and dimensions of a radiation alteration device mayinfluence other properties of a modified branch radiation beam B_(a)which is output from the radiation alteration device. For example, theshape and dimensions of a radiation alteration device may influence thedegree of spatial scrambling of rays 127 which form the branch radiationbeam B_(a) which is caused by the radiation alteration device.

As was described above with reference to FIG. 5 some embodiments of aradiation alteration device may include at least one bend. Including abend in a radiation alteration device may cause a change in the angularspread of rays which form a branch radiation beam B_(a). As a ray passesalong a bend in a radiation device the angle which the ray forms with anoptical axis which extends along the cross-sectional centre of theradiation alteration device may change and thus the angle which each rayforms with the optical axis is no longer conserved as the rays propagatealong the radiation alteration device. As a result the range of angleswhich the rays form with the optical axis at the second opening of theradiation alteration device may be increased by the presence of a bendin the radiation alteration device. For example, the maximum angle whichthe rays form with the optical axis may be increased by an amount whichis approximately equal to the bend angle α of a bend in the radiationalteration device. An increase in the range of angles which the raysform with the optical axis may increase the divergence of the modifiedbranch radiation beam B_(a) which is output from the radiationalteration device and may therefore cause a further increase in theetendue of the modified branch radiation beam B_(a).

An increase in the range of angles which the rays form with the opticalaxis of a radiation alteration device also causes an increase in therange of grazing angles with which each ray is incident on the internalsurface of the radiation alteration device. Increasing the range ofgrazing angles with which each ray is incident on the internal surfaceof the radiation alteration device may increase the variety of differentpaths with which rays propagate through the radiation alteration deviceand may therefore increase the spatial scrambling of the rays which iscaused by the radiation alteration device. Including one or more bendsin a radiation alteration device may therefore increase the spatialscrambling of rays of a branch radiation beam B_(a) which is caused by aradiation alteration device.

In the embodiments which have been described above and which have beenshown in the Figures, the branch radiation beam is focused to a focalpoint which lies on an optical axis of a radiation alteration devicebefore the branch radiation beam B_(a) enters the radiation alterationdevice. For example, in the embodiment which is shown in FIG. 8A thebranch radiation beam B_(a) is focused to a focal point 1081 which lieson the optical axis 1331 of the radiation alteration device 1011.However in some embodiments a branch radiation beam B_(a) may be focusedto a focal point which does not lie on an optical axis of a radiationalteration device.

FIG. 9 is a schematic illustration of a branch radiation beam B_(a)entering a radiation alteration device 1011 where a focal point 1081 ofthe branch radiation beam B_(a) does not lie on the optical axis 1331 ofthe radiation alteration device 1011. The optical axis 1331 of theradiation alteration device 1011 extends along the cross-sectionalcentre of the tube 1251. The optical axis 1331 extends into and out ofthe tube 1251 through the first 1211 and second 1221 openings. As isshown in FIG. 9 the branch radiation beam B_(a) is focused to a focalpoint 1081 which does not lie on the optical axis 1331. The amount bywhich the focal point 1081 is displaced from the optical axis 1331 maybe quantified with an off-axis angle β. The off-axis angle β is theangle which is formed between the optical axis 1331 and a line 1311which extends between the focal point 1081 and a position 1312 on theoptical axis 1331 at which the optical axis 1331 passes through thefirst opening 1211. In some embodiments the off-axis angle β may beapproximately the same as or is greater than the half divergence θ ofthe branch radiation beam B_(a) which enters the radiation alterationdevice 1011.

Focusing a branch radiation beam B_(a) to a focal point which does notlie on an optical axis 1331 of the radiation alteration device (as isshown in FIG. 9) may increase the range of angles which the rays 127form with the optical axis and may increase the range of grazing angleswith which the rays are incident on the internal surface of theradiation alteration device. Focusing a branch radiation beam to a focalpoint which does not lie on an optical axis of a radiation alterationdevice may therefore increase the divergence of the modified branchradiation beam B_(a) which is output from the radiation alterationdevice and may increase the spatial scrambling of the rays which iscaused by the radiation alteration device. In particular, focusing abranch radiation beam B_(a) to a focal point which does not lie on anoptical axis and which has an off-axis angle β which is approximatelythe same as or is greater than the half divergence θ of the branchradiation beam B_(a) which enters the radiation alteration device, mayresult in a desirable amount of spatial scrambling in the radiationalteration device 1011.

Additionally or alternatively the range of angles which the rays formwith the optical axis and the range of grazing angles with which therays are incident on the internal surface of a radiation alterationdevice may be increased by increasing the divergence of the branchradiation beam B_(a) which enters the first opening of the radiationalteration device. For example, the focal length of one or more focusingoptics (e.g. the first focusing optic 107 shown in FIG. 4) which areconfigured to focus the branch radiation beam B_(a) before the branchradiation beam B_(a) enters the radiation alteration device may bedecreased so as to increase the divergence of the branch radiation beamB_(a) which enters the radiation alteration device.

Embodiments have been described above in which a range of grazing angleswith which rays 127 of a branch radiation beam B_(a) are incident on aninternal surface of a radiation alteration device may be increased.However the reflectivity of an internal surface of a radiationalteration device may be a strong function of the grazing angle withwhich radiation is incident on the internal surface. In particular thereflectivity of an internal surface of a radiation alteration device maydecrease with increasing grazing angles. An increase in the grazingangles with which radiation is incident on an internal surface of aradiation alteration device may therefore lead to a decrease in theamount of radiation which is reflected at an internal surface of aradiation alteration device and may lead to an increase in an amount ofradiation which is lost from a branch radiation beam B_(a) as itpropagates along a radiation alteration device.

In some embodiments the internal surface of a radiation alterationdevice may comprise a substrate (e.g. a silicon substrate) on which areflective coating is disposed. For example, a smooth coating ofruthenium coating may be disposed on a substrate so as to form theinternal surface of a radiation alteration device. The amount of EUVradiation which is lost due to absorption during a reflection from aruthenium coating may be approximately proportional to the grazing anglewith which the EUV radiation is incident on the ruthenium coating. Forexample, approximately 0.06% of EUV radiation may be lost due toabsorption during a reflection from a ruthenium coating per milliradianof grazing angle.

In some embodiments coatings other than ruthenium may be used. Forexample, a coating comprising molybdenum may be used. In otherembodiments a multilayer coating may be used comprising alternatinglayers of a first and second material. The first material may, forexample, be molybdenum. The second material may, for example, besilicon. In some embodiments one or more layers of a third material maybe interspersed with the first and second materials. For example, alayer of a third material may be positioned between each layer of thefirst and second materials. The third material may, for example, be B₄C.In some embodiments the first material may be Mo₂C and the secondmaterial may be silicon. Radiation alteration devices comprising amultilayer coating may, for example, be suitable for receiving andreflecting EUV radiation at grazing incidence angles which are greaterthan about 200 miliradians.

In some embodiments the divergence of a branch radiation beam B_(a) asit enters a radiation alteration device, the bend angle α of a bend inthe radiation alteration device and/or the extent to which a focal pointof the branch radiation beam B_(a) is located away from an optical axisof the radiation alteration device, may be limited so as to limit themaximum grazing angle with which rays 127 are incident on an internalsurface of the radiation alteration device. Since the amount ofradiation which is absorbed during a reflection at the internal surfaceincreases with increasing grazing angle, limiting the maximum grazingangle may limit the amount of radiation which is lost from the branchradiation beam B_(a) due to absorption. In some embodiments, the bendangle α of a bend in a radiation alteration device may be less thanapproximately 5 degrees. In some embodiments the bend angle α of a bendin a radiation alteration device may be less than about 2 degrees.

In some embodiments the bend angle α may be approximately equal to orgreater than the half divergence θ of the branch radiation beam B_(a) asit enters the radiation alteration device. In some embodiments the bendangle α may be sufficiently large that no direct line of sight isprovided through the radiation alteration device. In such embodimentsthere is no optical path along which radiation can propagate through theradiation alteration device without undergoing a reflection at theinternal surface of the radiation alteration device.

An embodiment of a radiation alteration device may comprise a tubehaving a length of approximately 1 m and a diameter of approximately 1mm. The tube may include a bend angle of approximately 10 milliradians.The radiation alteration device may include an internal surfacecomprising a smooth ruthenium coating. In such an embodiment if a branchradiation beam B_(a) is focused to a focal point positioned on anoptical axis of the radiation alteration device such that the branchradiation beam B_(a) enters the radiation alteration device with a halfdivergence θ of approximately 10 milliradians then approximately 5% ofthe branch radiation beam B_(a) may be lost to absorption at theinternal surface.

An alternative embodiment of a radiation alteration device may comprisea tube having a length of approximately 1 m and a diameter ofapproximately 4 mm. The tube may include a bend angle of approximately20 miliradians. The radiation alteration device may include an internalsurface comprising a smooth ruthenium coating. In such an embodiment abranch radiation beam B_(a) may be focused to a focal point which is notpositioned on an optical axis of the radiation alteration device suchthat the off-axis angle β is approximately 20 miliradians. The branchradiation beam B_(a) may enter the radiation alteration device with ahalf divergence θ of approximately 20 milliradians. In such anembodiment the divergence of the branch radiation beam B_(a) may beincreased by the radiation alteration device such that the halfdivergence of the branch radiation beam B_(a) which is output from theradiation alteration device is approximately 60 miliradians. Theradiation alteration device may increase the etendue of the branchradiation beam B_(a) such that the etendue of the branch radiation beamB_(a) which is output from the radiation alteration device is of theorder of 1×10⁸ times larger than the etendue of the branch radiationbeam B_(a) which is input to the radiation alteration device. Such anembodiment of a radiation alteration device may result in approximately15% of the power of the branch radiation beam B_(a) being lost toabsorption at the internal surface.

In other embodiments the branch radiation beam B_(a) may enter theradiation alteration device with a half divergence θ of approximatelyother than 20 milliradians. In some embodiments, the half divergence θmay, for example, be greater than about 10 milliradians. In someembodiments, the half divergence θ may, for example, be less than about100 milliradians. In some embodiments, the radiation alteration devicemay not include a bend and the divergence of the radiation which exitsthe radiation alteration device may be substantially the same as thedivergence of the radiation which enters the radiation alterationdevice.

The amount of radiation which is lost to absorption at the internalsurface may, for example, be decreased by decreasing the divergence ofthe branch radiation beam B_(a) as it enters the radiation alterationdevice, decreasing the bend angle of the bend in the radiationalteration device and/or altering the dimensions of the radiationalteration device so as to decrease the number of reflections which eachray of the branch radiation beam B_(a) undergoes at the internalsurface. However as was described above, increases in the divergence ofthe branch radiation beam B_(a), increases in the bend angle α of a bendin a radiation alteration device and/or increasing a number ofreflections which each ray undergoes at the internal surface may bringabout other advantageous effects. The dimensions of a radiationalteration device and the focusing of a branch radiation beam B_(a)prior to input of the branch radiation beam B_(a) to a radiationalteration device may therefore be selected so as to bring about anadvantageous modification of the branch radiation beam B_(a) whilstreducing the loss of radiation caused by absorption of the branchradiation beam B_(a) at the internal surface of the radiation alterationdevice.

It will be appreciated from the discussion provided above that aradiation alteration device may be configured with a variety ofdifferent shapes and dimensions each of which influences themodification of a branch radiation beam B_(a) which is caused by theradiation alteration device. It will be appreciated that the shape anddimensions of a radiation alteration device may be tailored to specificapplications so as to provide a desired modification of a branchradiation beam B_(a).

As was described above it is desirable to reduce the amount of radiationwhich is lost from a branch radiation beam B_(a) through absorption ofradiation at the internal surface of a radiation alteration device. Theloss of radiation from the branch radiation beam B_(a) may increase ifthe internal surface of the radiation alteration device becomescontaminated. For example, during use particles and/or molecules may bedeposited on the internal surface of the radiation alteration device.For example, carbon molecules may be deposited on the internal surfaceof the radiation alteration device, thereby leading to the growth of acarbon layer on the internal surface. Contamination which is depositedon the internal surface of a radiation alteration device may absorband/or scatter radiation from the branch radiation beam B_(a) thereforecausing radiation to be lost from the branch radiation beam B_(a). Inorder to reduce contamination of the internal surface of a radiationalteration device it may be desirable to clean the internal surface.

FIG. 10 is a schematic illustration of an embodiment of a radiationalteration device 1012 which provides for cleaning of an internalsurface 1232 of the radiation alteration device 1012. The radiationalteration device 1012 comprises a tube 1252 having a first section 1252a and a second section 1252 b. The first and second sections 1252 a,1252 b of the tube define an internal surface 1232 of the tube which issubstantially reflective to EUV radiation. The first section 1252 a ofthe tube includes a first opening 1212 arranged to receive a branchradiation beam B_(a). The second section 1252 b of the tube includes asecond opening configured to output a modified branch radiation beam Ba.The second section 1252 b of the tube is arranged to receive radiationfrom the first section 1252 a of the tube. The first and second sections1252 a, 1252 b of the tube are arranged so as to form a gap 1412 betweenthe first and second sections 1252 a, 1252 b through which gas may enteror leave the tube 1252. The radiation alteration device 1012 furthercomprises gas supplies 1432 configured to inject gas 1452 through thegap 1412 and into the tube 1252.

The gas 1452 may, for example, comprise hydrogen. The hydrogen gas whichis injected into the tube 1252 is irradiated with EUV radiation from thebranch radiation beam B_(a) which may result in the formation ofhydrogen radicals. Hydrogen radicals are highly reactive and if broughtinto contact with contamination which may be deposited on the internalsurface 1232 the hydrogen radicals may react with the contamination tocause a gaseous compound. The gaseous compound formed from a reactionbetween hydrogen radicals and contamination may flow out of the tube1252, for example through the first opening 1212 or the second opening1222, thereby removing the contamination from the tube 1252. Theinjection of hydrogen gas into the tube therefore serves to clean theinternal surface 1232 of the tube 1252.

The first and second sections 1252 a, 1252 b of the tube 1252 arearranged relative to each other such that radiation which enters thetube 1252 through the first opening 1212 does not exit the tube 1252through the gap 1412. As is shown in FIG. 10 a branch radiation beamB_(a) comprising a series of diverging rays 127 enters the tube 1252through the first opening 1212. The rays 127 undergo reflections (notshown in FIG. 10) at the internal surface 1232 such that the rays 127exit the first section 1252 a of the tube 1252 propagating in directionssuch that they are incident on the internal surface 1232 of the secondsection 1252 b and are not incident on the gap 1412. The first andsecond sections 1252 a, 1252 b are therefore arranged relative to eachother such that no radiation is lost through the gap 1412.

In the embodiment which is shown in FIG. 10 the second section 1252 b ofthe tube is arranged at an angle with respect to the first section 1252a of the tube so as to form a bend in the tube 1252. However in otherembodiments the second section 1252 b may extend in substantially thesame direction as the first section 1252 a of the tube such that thefirst and second sections do not form a bend.

As was described above a radiation alteration device may be configuredso as to cause rays 127 which form a branch radiation beam B_(a) to bespatially scrambled by the radiation alteration device. In someembodiments it may be desirable to further introduce temporal variationto the modified branch radiation beam B_(a) which is output from aradiation alteration device.

FIG. 11 is a schematic illustration of an embodiment of a radiationalteration device 1013 which is configured to introduce temporalvariations to a modified branch radiation beam B_(a) which is outputfrom the radiation alteration device 1013. The radiation alterationdevice 1012 comprises a tube 1253 having a first opening arranged toreceive a branch radiation beam B_(a) and a second opening 1223 arrangedto output a modified branch radiation beam B_(a). The tube 1253 isdefined by an internal surface 1233 which is substantially reflective toEUV radiation. The radiation alteration device further comprises anactuator 1453 operable to actuate the tube 1253 so as to cause the tube1253 to undergo an oscillatory motion. For example, the tube 1253 may berepeatedly moved between a first position (indicated by the solid linein FIG. 11) and a second position (indicated by the dashed line in FIG.11). Movement between the first and second positions may, for example,comprise pivoting the tube 1253 about a pivot point 1473. In someembodiments the actuator 1453 may be operable to actuate the tube 1253so as to cause the tube 1253 to undergo a circular motion. For example,the first opening 1213 of the tube 1253 may undergo a circular motion inthe x-y plane.

Movement of the tube 1253 may cause a change in position at which therays 127 of the branch radiation beam B_(a) are output from the secondopening 1223 of the radiation alteration device 1253 and/or may cause achange in the directions in which the rays 127 propagate as they areoutput from the second opening 1223. Movement of the tube 1253 thereforecauses a temporal scrambling of the rays 127 in addition to the spatialscrambling of the rays 127 which has been described above. Consequentlythe angular and spatial intensity profile of the modified branchradiation beam B_(a) which is incident on the field facet mirror 10 mayoscillate with time.

During exposure of a substrate W in a lithographic apparatus LA_(a) eachtarget portion of the substrate W is exposed to radiation for a givenexposure time. In general it is desirable to expose a target portion ofa substrate W to a desired dose of radiation during an exposure timeperiod. It is therefore the integral of the intensity of radiation towhich the target portion is exposed during an exposure time period whichis important rather than the nature of any variations in the intensityof radiation which occur during an exposure time period. In order toensure that each target portion of a substrate W is exposed toapproximately the same dose of radiation it is desirable that the timeperiod of the oscillatory motion of the tube 1253 is less than theexposure time of a target portion of the substrate W. In someembodiments, the exposure time of a target portion of a substrate W maybe approximately 1 ms. In such embodiments the frequency of theoscillatory motion of the tube 1253 may be greater than about 1 kHz suchthat the time period of the oscillatory motion is less than the exposuretime. In some embodiments the frequency of the oscillatory motion may beabout 5 kHz or may be greater than about 5 kHz.

Introducing a temporal scrambling of the rays 127 which are output fromthe radiation alteration device 1013 may be advantageous for the samereasons as were described above with reference to a spatial scramblingof the rays 127. In some embodiments introducing a temporal scramblingof the rays 127 may allow the dimensions of a radiation alterationdevice to be adjusted so as to reduce the loss of radiation from thebranch radiation beam B_(a) due to absorption at the internal surface ofthe radiation alteration device.

The degree of spatial scrambling of rays 127 which is caused by aradiation alteration device increases with the number of reflectionswhich the rays 127 undergo at the internal surface of the radiationalteration device. The number of reflections which the rays 127 undergomay be approximately proportional to θL/D where θ is the half divergencewith which a branch radiation beam B_(a) enters a radiation alterationdevice, L is the length of the radiation alteration device and D is thediameter of a tube which forms the radiation alteration device. Thedegree of spatial scrambling may therefore be increased by increasingthe divergence of the branch radiation beam B_(a), increasing the lengthL of the radiation alteration device and/or decreasing the diameter ofthe radiation alteration device. However the amount of radiation whichis lost from a branch radiation beam B_(a) due to absorption at theinternal surface of a radiation alteration device may be approximatelyproportional to θ² L/D. Altering the dimensions of a radiationalteration device and/or the divergence of the branch radiation beamB_(a) in order to increase the degree of spatial scrambling of the rays127 may therefore result in an increase in the amount of radiation whichis lost from the branch radiation beam B_(a) due to absorption.

Introducing a temporal scrambling of the rays 127 which are output fromthe radiation alteration device 1013 may allow the degree of spatialscrambling which is caused by the radiation alteration device 1013 to bedecreased whilst still achieving the advantageous effects which areassociated with scrambling the rays 127. Introducing a temporalscrambling of the rays 127 may therefore allow the length L of theradiation alteration device 1013 to be decreased, the divergence of thebranch radiation beam B_(a) to be decreased and/or the diameter D of thetube 1253 to be increased, thereby advantageously decreasing the amountof radiation which is lost from the branch radiation beam B_(a) due toabsorption whilst still achieving the advantageous effects which areassociated with a scrambling the rays 127.

Whilst an oscillatory motion of the tube 1253 has been described abovein order to introduce a temporal scrambling of the rays 127, in someembodiments one or more other optical components may be forced toundergo an oscillatory motion in order to introduce a temporalscrambling. For example, one or more actuators may be configured tocause one or more of the second focusing optical 109 and/or the thirdfocusing optic 111 to undergo an oscillatory motion so as to introduce atemporal scrambling to the rays 127.

As has been described above, in some embodiments the modified branchradiation beam B_(a) which is output from a radiation alteration deviceis focused (e.g. with the second focusing optic 109 and the thirdfocusing optic 111 shown in FIG. 4) to an intermediate focus IF locatedat an opening 8 in an enclosing structure of a lithographic apparatusLA_(a). FIGS. 12A and 12B are schematic illustrations of two alternativefocusing schemes which may be used to focus the modified branchradiation beam B_(a) output from a radiation alteration device to anintermediate focus IF so as to illuminate a field facet mirror 10.

For ease of illustration and explanation the second focusing optic 109and the third focusing optic 111 are shown in FIGS. 12A and 12B as beingtransmissive focusing optics. However in practice the focusing schemesillustrated in FIGS. 12A and 12B may be implemented using reflectivefocusing optics as will be understood by a person skilled in the art.The two focusing schemes which are shown in FIGS. 12A and 12B differfrom each other in that the distances between the various components aredifferent. In each of FIGS. 12A and 12B, the distance between a secondopening 122 of a radiation alteration device from which a modifiedbranch radiation beam B_(a) is output and the second focusing element109 is labeled L₁. The distance between the second focusing element 109and the third focusing element 111 is labeled L₂ and the distancebetween the second focusing element 111 and the intermediate focus IF islabeled L₃. The second focusing optic 109 has a focal length f₂ which isless than zero. The third focusing optic 111 has a focal length f₃ whichis greater than zero. The path of the modified branch radiation beamB_(a) from the second opening 122 of the radiation alteration device toa far field plane in which the field facet mirror 10 lies is shown inFIGS. 12A and 12B with solid and dotted lines 127 which representdifferent rays 127 which form the modified branch radiation beam B_(a).

In the focusing scheme which is shown in FIG. 12A, the second focusingoptic 109 and the third focusing optic 111 are configured to form animage of the second opening 122, from which the modified branchradiation beam B_(a) is output, on the field facet mirror 10. An imageof the second opening 122 is formed on the field facet mirror 10 byensuring that the distance L₁ between the second opening 122 and thesecond focusing optic 109 is sufficiently small that the modifiedradiation beam B_(a) reaches the second focusing optic 109 before therays 127 have substantially overlapped with each other. This may beachieved if the distance L₁ between the second opening 122 and thesecond focusing optic 109 is much less than the diameter of the secondopening 122 divided by the divergence of the modified branch radiationbeam B_(a) at the second opening. Since the focusing scheme which isshown in FIG. 12A forms an image of the second opening 122 on the fieldfacet mirror 10, the spatial intensity profile of the radiation which isincident on the field facet mirror 10 depends on the spatial intensityprofile of the modified branch radiation beam B_(a) at the secondopening 122.

Whilst, it has been stated above that the focusing scheme which is shownin FIG. 12A is configured to form an image of the second opening 122 onthe field facet mirror 10, an in focus image of the second opening 122may, in practice, be formed in a plane which lies between theintermediate focus and the field facet mirror 10. The image which isformed on the field facet mirror 10 may therefore be out of focus.However, despite the image which is formed on the field facet mirror 10being out of focus, the intensity profile of the radiation which isincident on the field facet mirror 10 may still be predominantlydependent on the spatial intensity profile of the modified branchradiation beam B_(a) at the second opening 122.

In contrast to the focusing scheme which is shown in FIG. 12A, in thefocusing scheme which is shown in FIG. 12B the distance L₁ between thesecond opening 122 and the second focusing optic 109 is much greaterthan in the focusing scheme which is shown in FIG. 12A. Consequently animage of the second opening 122, from which the modified branchradiation beam B_(a) is output, is no longer formed on the field facetmirror 10 but is instead formed close to the intermediate focus IF.Since an image of the second opening 122 is formed close to theintermediate focus IF, the radiation which is incident on the fieldfacet mirror 10 depends on the angular intensity profile of the modifiedbranch radiation beam B_(a) at the second opening 122.

Typically the spatial intensity profile of the branch radiation beamB_(a) at the second opening 122 is more homogenous than the angularintensity profile of the branch radiation beam B_(a) at the secondopening 122. Consequently the focusing scheme which is shown in FIG. 12Aadvantageously results in a spatial intensity profile at the field facetmirror 10 which is more homogenous than the spatial intensity profilewhich is formed at the field facet mirror 10 due to the focusing schemewhich is shown in FIG. 12B.

As was described above, in the embodiments which are shown in FIGS. 12Aand 12B the second focussing optic 109 has a negative focussing power(focal length of less than zero) and the third focussing optic 111 has apositive focussing power (focal length of greater than zero). FIG. 12Cis a schematic illustration of a still further embodiment of a focussingscheme which may be used to focus a modified branch radiation beam B_(a)which is output from a radiation alteration device so as to illuminate afield facet mirror 10. The focusing scheme which is shown in FIG. 12Ccomprises a second focusing optic 109′ and a third focusing optic 111′which both have a positive focusing power (focal length of greater thanzero). The second focusing optic 109′ is configured to form a magnifiedimage of the second opening 122 of a radiation alteration device at animage plane 110′. The third focusing optic 111′ is configured to focusthe image which is formed at the image plane 110′ on to the field facetmirror 10. The focusing scheme which is shown in FIG. 12C thereforefocuses an image of the second opening 122 of a radiation alterationdevice onto the field facet mirror 10.

As was described above the spatial intensity profile of a branchradiation beam B_(a) at the second opening 122 is typically morehomogenous than the angular intensity profile of the branch radiationbeam B_(a). Forming an image of the second opening 122 on the fieldfacet mirror 10 may therefore advantageously result in a spatialintensity profile at the field facet mirror 10 which is relativelyhomogenous. The focusing arrangement which is shown in FIG. 12C andwhich includes two focusing optics both having a positive focusing powermay advantageously allow the image of the second opening 122 which isformed at the field facet mirror 10 to be substantially in focus at thefield facet mirror 10. That is, the focusing scheme of FIG. 12C mayadvantageously allow an image of the second opening 122 to be formed ina focal plane which is substantially coincident with the plane in whichthe field facet mirror 10 lies. This is in contrast to the focusingschemes of FIGS. 12A and 12B in which the focal plane of an image of thesecond opening 122 lies at or near to the intermediate focus IF (FIG.12B) or between the intermediate focus IF and the field facet mirror 10(FIG. 12A). As was described above, forming an in-focus image of thesecond opening 122 substantially at the field facet mirror 10 mayadvantageously increase the spatial homogeneity of radiation which isincident on the field facet mirror (when compared to forming an imagewhose focal plane lies before the field facet mirror 10). The focusingscheme which is shown in FIG. 12C may, in some embodiments, thereforeprovide a spatial intensity distribution at the field facet mirror 10which is more homogenous than a spatial intensity distribution which isprovided at the field facet mirror 10 by the focusing schemes shown inFIGS. 12A and 12B.

As is shown in FIG. 12C the second 109′ and third focusing optics 111′are configured to focus rays 127 of the branch radiation beam B_(a) suchthat all of the rays 127 pass substantially through an intermediatefocus IF. Whilst the intermediate focus IF which is shown in FIG. 12C isreferred to as a focus, in the embodiment of FIG. 12C the rays 127 arenot focused to a single focal point and the branch radiation beam B_(a)is not in focus at the intermediate focus IF. In the embodiment of FIG.12C the intermediate focus IF is not therefore a focal point and merelyrepresents a region through which all rays 127 pass. The region throughwhich the rays 127 pass may, for example, coincide with an opening 8 inan enclosing structure of a lithographic apparatus LA.

The term “intermediate focus” is used throughout this document to referto a region through which rays 127 of a radiation beam (e.g. a branchradiation beam B_(a)) are directed to pass. An intermediate focus IFmay, in some embodiments, be a focal point. In other embodiments anintermediate focus IF may be a region having a non-zero area. Anintermediate focus IF may coincide with an opening 8 in an enclosingstructure of a lithographic apparatus LA such that rays 127 which passthrough the intermediate focus IF also pass through the aperture 8 andinto the lithographic apparatus LA.

A focusing scheme of the type which is depicted in FIG. 12C may beimplemented in a number of different ways. For example, the focallengths of the second 109′ and third 111′ focusing optics and theseparations between the second opening 122, the focusing optics 109′,111′, the intermediate focus IF and the field facet mirror 10 may beconfigured differently in different embodiments.

In an embodiment, which is presented by way of example only, the secondopening 122 of a radiation alteration device 122, having a diameter ofapproximately 4 mm, may be imaged on to a field facet mirror 10 using afocusing scheme of the type shown in FIG. 12C. In such an embodiment thedistance L₁ between the second opening 122 and the second focusing optic109′ may be approximately 109 mm. The focal length f₂ of the secondfocusing optic 109′ may be approximately 104 mm. The distance L₂ betweenthe second focusing optic 109′ and the third focusing optic 111′ may beapproximately 2.5 m. The focal length f₃ of the third focusing optic111′ may be approximately 190 mm. The distance L₃ between the thirdfocusing optic 111′ and the intermediate focus IF may be approximately209 mm. A distance L₄ between the intermediate focus IF and the fieldfacet mirror 10 may be approximately 1 m.

For ease of illustration, in the representation of the focusing schemewhich is depicted in FIG. 12C the second focusing optic 109′ and thethird focusing optic 111′ are depicted as being transmissive focusingoptics. However, it will be understood by those skilled in the art thatthe focusing scheme which is depicted in FIG. 12C may be implementedusing reflective optics. For example, the second focusing optic 109′and/or the third focusing optic 111′ may be mirrors. In someembodiments, the second focusing optic 109′ and/or the third focusingoptic 111′ may be implemented as grazing-incidence mirrors. The secondfocusing optic 109′ and/or the third focusing optic 111′ may have areflective surface which has a substantially ellipsoidal shape. In someembodiments the second focusing optic 109′ and/or the third focusingoptic 111′ may have a reflective surface which has an optimizedfree-form shape which is not substantially ellipsoidal.

The embodiments depicted in FIGS. 12A-12C each comprise two focusingoptics. In other embodiments, a suitable focusing scheme may be realizedusing only a single focusing optic. In the embodiments of FIGS. 12A-12Cthe second opening 122 of a radiation alteration device may be imagedonto an image plane with a relatively large magnification ratio. Inorder to achieve such a magnification using a single focusing optic, thesingle focusing optic may need to be positioned very near to the secondopening 122. Such an implementation may be impractical when using areflective focusing element which receives radiation at grazingincidence angles. The use of a plurality of focusing optics,advantageously allows the focusing optics to be located at a greaterdistance from the second opening 122. This may allow grazing incidencereflective optics to be used. However, in alternative embodiments themagnification ratio with which the second opening 122 of the radiationalteration device is imaged onto an image plane may be smaller than thatshown in FIGS. 12A-12C. This may allow for a practical implementation ofthe focusing schemes using a single focusing optic.

FIG. 12D is a schematic illustration of an embodiment of the focusingscheme of FIG. 12C in which the second 109′ and third 111′ focusingoptics are implemented as grazing-incidence mirrors each having asubstantially ellipsoidal reflective surface. A branch radiation beamB_(a) is output from a second opening 122 of a radiation alterationdevice and is incident on the second reflective focusing optic 109′. Thesecond reflective focusing optic 109′ has a first focal point 1091 and asecond focal point 1092. The first focal point 1091 lies substantiallyin a plane which is defined by the second opening 122 of the radiationalteration device. The second focal point 1092 lies in an image plane110′ in which an image of the second opening 122 is formed.

The third reflective focusing optic 111′ has a first focal point 1111and a second focal point 1112. The first focal point 1111 of the thirdfocusing optic 111′ is substantially coincident with the second focalpoint 1092 of the second focusing optic 111′. The second focal point1112 of the third focusing optic 111′ lies substantially in a planewhich is defined by the field facet mirror 10. The second focusing optic111′ therefore forms an image of the image plane 110′ at the field facetmirror 10. The second focusing optic 111′ also serves to direct thebranch radiation beam B_(a) through the intermediate focus IF, which inthe embodiment of FIG. 12D comprises a region through which all rays ofthe branch radiation beam B_(a) are directed. The intermediate focus IFmay, for example, coincide with an opening 8 in an enclosing structureof a lithographic apparatus LA.

The reflective focusing optics 109′, 111′ of the embodiment of FIG. 12Deach comprise a reflective surface having a substantially ellipsoidalshape. Ellipsoidal reflective surfaces are configured to image radiationfrom a first focal point to a second focal point. For example, thesecond focusing optic 109′ images radiation from its first focal point1091 to its second focal point 1092. However in the embodiment of FIG.12D the ellipsoidal reflective surfaces are arranged to image a plane,as opposed to a point. For example, the second focusing optic 109′ isarranged to form an image of a plane defined by the second opening 122in an image plane 110′. As a result of forming an image of a plane withan ellipsoidal reflective surface the image which is formed in the imageplane 110′ will be blurred, for example, due to comatic aberrationand/or other higher order aberrations. Similarly the image which isformed at the field facet mirror 10 will be blurred.

Forming a blurred image at the field facet mirror may, in someembodiments, be acceptable and may not cause any substantialdisadvantageous effects. However in other embodiments aberrations (e.g.comatic aberrations) may cause disadvantageous effects. For example,aberrations may cause some radiation to not pass through theintermediate focus IF which may result in radiation being lost from thebranch radiation beam B_(a) at the intermediate focus IF. Additionallyor alternatively aberrations may cause the cross-sectional shape of thebranch radiation beam B_(a) at the field facet mirror 10 to be differentto the cross-sectional shape of the field facet mirror 10. A mismatchbetween the cross-sectional shapes of the branch radiation beam B_(a)and the field facet mirror 10 may result in radiation being lost fromthe branch radiation beam B_(a) at the field facet mirror 112.

Aberrations which are present in an image formed at the field facetmirror 10 may be reduced by using reflective elements which areconfigured generally in the form of a Wolter telescope. A Woltertelescope is an arrangement of grazing incidence mirrors which areconfigured to form an image of a source plane at an image plane. Thesource plane which is imaged by a Wolter telescope may be located at alarge distance from the Wolter telescope (which may be considered to bean infinite distance) and the image plane onto which the source plane isimaged may be located relatively close to the Wolter telescope.Alternatively the source plane which is imaged by a Wolter telescope maybe located relatively close to the Wolter telescope and the image ontowhich the source plane is imaged may be located at a large distance fromthe Wolter telescope (which may be considered to be an infinitedistance).

Typically the term “Wolter telescope” may be used to refer to anarrangement comprising a reflective element having an ellipsoidal shapeand a reflective element having a parabolic shape. The reflectiveelements may be configured to image a nearby plane onto a plane atinfinity or a plane at infinity onto a nearby plane. The Woltertelescopes described herein may differ from conventional Woltertelescopes in that they are configured to image a large conjugate plane(not at infinity) onto a nearby plane or vice versa. The Woltertelescopes described herein may, for example, comprise a firstreflective element having an ellipsoidal shape and a second reflectiveelement having an ellipsoidal shape which is close to being parabolic.

FIG. 12E is a schematic illustration of an embodiment in which an imageof the second opening 122 of a radiation alteration device is imagedonto a field facet mirror 112 using optics arranged in the form of aWolter telescope. In the embodiment which is shown in FIG. 12E thesecond 109′ and third 111′ focusing optics each comprise a Woltertelescope comprising two reflective elements. The Wolter telescope whichforms the second focusing optic 109′ comprises a first reflectiveelement 109 a′ and a second reflective element 109 b′. The Woltertelescope which forms the third focusing optic 111′ comprises a thirdreflective element 111 a′ and a fourth reflective element 111 b′.

The first 109 a′ and second 109 b′ reflective elements together form aWolter telescope which is configured to image a plane which is locatedrelatively close to the Wolter telescope onto a plane which is locatedat an infinite distance from the Wolter telescope. The image plane 110′is not at an infinite distance from the Wolter telescope which forms thesecond focusing optic 109′. Consequently the second opening 122 will notbe perfectly imaged onto the image plane 110′. However, the ratio of thedistance between the second opening 122 and the second focusing optic109′ and the distance between the second focusing optic 109′ and theimage plane 110′ may be sufficiently small that an image of the secondopening 122 is formed at the image plane 110′ with relatively fewaberrations. For example, using a Wolter telescope to form an image ofthe second opening 122 at the image plane 110′ (as shown in FIG. 12E)may reduce any aberrations (e.g. comatic aberrations) present in theimage, when compared to using ellipsoidal mirrors to form the image (asshown in FIG. 12D).

Similarly to the Wolter telescope which is formed from the first 109 a′and second 109 b′ reflective elements, the third 111 a′ and fourth 111b′ reflective elements together form a Wolter telescope which isconfigured to image a plane which is located relatively close to theWolter telescope onto a plane which is located at an infinite distancefrom the Wolter telescope. The ratio of the distance between the imageplane 110′ and the third focusing optic 111′ and the distance betweenthe image plane 111 a′ and the field facet mirror 10 may be sufficientlysmall that an image of the image plane 110′ is formed at the field facetmirror 10 with relatively few aberrations. For example, aberrationswhich are present in the image which is formed at the field facet mirrormay be reduced when using a Wolter telescope to form the image (as isshown in FIG. 12E), when compared to using ellipsoidal mirror to formthe image (as shown in FIG. 12D).

As was described above in connection with the embodiment shown in FIG.12E optical elements arranged in the form of one or more Woltertelescopes may advantageously reduce aberrations in an image of thesecond opening 122 which is formed at the field facet mirror 10. In theembodiment which is shown in FIG. 12E the Wolter telescopes which formthe second 109′ and third 111′ focusing optics are both of the form of atype-III Wolter telescope. In other embodiments a (rotationallysymmetric) type-I and/or a type-II Wolter telescope may be used to forman image of the second opening 122 at the field facet mirror 10.

Embodiments of a radiation alteration device have been described abovein the context of a radiation alteration device which is arranged toreceive a branch radiation beam B_(a) prior to the branch radiation beamB_(a) being provided to a lithographic apparatus LA_(a). Howeverembodiments of a radiation alteration device may additionally oralternatively be advantageously used for other purposes and at differentpositions in a lithographic system than has been described above. Forexample, as was described above with reference to FIG. 1 a lithographicsystem LS may include a beam delivery system BDS comprising beamsplitting optics configured to split a main radiation beam B into aplurality of branch radiation beams B_(a)-B_(n). As will be described infurther detail below a radiation alteration device may additionally oralternatively be advantageously used to modify a main radiation beam Bprior to the main radiation beam B being provided to beam splittingoptics.

FIG. 13 is a schematic illustration of a beam splitting apparatus 2001arranged downstream of an embodiment of a radiation alteration device1014. The radiation alteration device 1014 is arranged to receive a mainradiation beam B. The main radiation beam B may, for example, be outputfrom a radiation source SO comprising at least one free electron laserFEL. The main radiation beam B is incident on a fourth focusing optic2003 and a fifth focusing optic 2004 prior to being provided to theradiation alteration device 1014. For ease of illustration the fourthfocusing optic 2003 and the fifth focusing optic 2004 are depicted inFIG. 13 as transmissive optics however in practice the fourth focusingoptic 2003 and the fifth focusing optic 2004 may be implemented asreflective optics as will be well understood by a person skilled in theart.

The fourth focusing optic 2003 and the fifth focusing optic 2004 aretogether configured to expand the main radiation beam B. The fourthfocusing optic 2003 is configured to introduce divergence into the mainradiation beam B so as to expand the cross-section of the main radiationbeam B. For example, the fourth focussing optic 2003 may be configuredto expand the cross-section of the main radiation beam B such that thecross-section of the main radiation beam B is approximately similar tothe cross-section of a first opening 1214 of the radiation alterationdevice 1014. In the example which is shown in FIG. 13, the radiationalteration device 1014 has a rectangular cross-section having a greaterextent in the x-direction than in the y-direction. The fourth focusingoptic 2003 may therefore be configured to introduce a larger divergencein the y-direction than in the x-direction so as to expand the mainradiation beam B to approximately match the rectangular cross-section ofthe radiation alteration device 1014.

In the example which is shown in FIG. 13, the fifth focusing optic 2004is configured to decrease the divergence of the main radiation beam Bprior to the main radiation beam B entering the radiation alterationdevice 1014 through the first opening 1214. The fifth focusing optic2004 may, for example, be configured to decrease the divergence of themain radiation beam B so as to reduce the grazing angle with whichradiation is incident on an internal surface of the radiation alterationdevice. As was described above with reference to other embodiments of aradiation alteration device, the amount of radiation which is lost froma radiation beam through absorption at an internal surface of aradiation alteration device increases with increases in the grazingangle with which radiation is incident on the internal surface. Thefifth focusing optic 2004 may therefore decrease the divergence of themain radiation beam B so as to reduce the amount of radiation which islost from the main radiation beam B due to absorption at the internalsurface of the radiation alteration device 1014.

The fourth focusing optic 2003 and the fifth focusing optic 2004 arepresented merely as examples of focusing optics which may be used toadapt the main radiation beam B prior to the main radiation beamentering a radiation alteration device 1014. In other embodiments moreor less focusing optics than are shown in FIG. 13 may be used. In someembodiments the focusing optics may be configured differently than theconfiguration shown in FIG. 13. In general any one or more focusingoptics may be used in order to adapt to the main radiation beam B suchthat the main radiation beam B has one or more desired properties uponentry to the radiation alteration device 1014. For example, one or morefocusing optics may be configured to expand the cross-section of themain radiation beam such that the cross-sectional size of the mainradiation beam B is approximately close to the cross-sectional size ofthe first opening 1214 of the radiation alteration device 1014 and maybe configured to cause the main radiation beam B to be diverging uponentry to the radiation alteration device such that the main radiationbeam B undergoes reflection at an internal surface of the radiationalteration device 1014.

As was described above with reference to other embodiments of aradiation alteration device arranged to receive a branch radiation beamB_(a), radiation which enters the first opening 1214 of the radiationalteration device 1014 undergoes multiple reflections at a reflectiveinternal surface of the radiation alteration device. The radiationalteration device 1014 is configured to output a modified main radiationbeam B at a second opening 1224 of the radiation alteration device 1014.

As will be appreciated from the discussion of other embodiments ofradiation alteration devices which was provided above, the radiationalteration device 1014 causes rays which form the main radiation beam Bto be spatially scrambled. The spatial scrambling of the rays which formthe main radiation beam B may increase the homogeneity of the intensityprofile of the main radiation beam B, such that the modified mainradiation beam B which is output from the second opening 1224 of theradiation alteration device 1014 has a more homogenous intensity profilethan the intensity profile of the main radiation beam which enters theradiation alteration device 1014 through the first opening 1214.Furthermore the modified radiation beam B which is output from thesecond opening 1224 of the radiation alteration device has across-sectional shape which corresponds to the cross-sectional shape ofthe second opening 1224 of the radiation alteration device 1014.

The modified main radiation beam B which is output from the radiationalteration device is incident on a beam splitting apparatus 2001. Thebeam splitting apparatus 2001 comprises a first reflective facet 2007 a,a second reflective facet 2007 b and a third reflective facet 2007 c.The reflective facets 2007 a-2007 c are each arranged to receivedifferent portions of the cross-section of the modified radiation beam Bwhich is output from the second opening 1224 of the radiation alterationdevice 1014 and reflect the different portions in different directions.As is shown in FIG. 13, the reflection of different portions of themodified radiation beam B in different directions causes the formationof separate branch radiation beams B_(a), B_(b) and B_(c). The branchradiation beams B_(a), B_(b), B_(c) may each be directed to differentlithographic apparatus. It may be desirable to split the main radiationbeam B into branch radiation beams B_(a)-B_(c) which have approximatelyequal cross-sections and powers such that the lithographic apparatus areprovided with branch radiation beams having substantially similarproperties.

As was explained above, the cross-sectional shape of the modified mainradiation beam B which is output from the radiation alteration device1014 is equivalent to the cross-sectional shape of the second opening1224 of the radiation alteration device 1014. In the embodiment which isshown in FIG. 13 the radiation alteration device 1014 is provided with amain radiation beam B having an approximately elliptical cross-section(after having been expanded by the focusing optics 2003, 2004). Theradiation alteration device 1014 has a rectangular cross-section suchthat the modified main radiation beam B which is output from therectangular second opening 1224 of the radiation alteration device 1014also has a rectangular cross-section. The radiation alteration device1224 therefore serves to convert the cross-sectional shape of the mainradiation beam B from being substantially elliptical to beingsubstantially rectangular. It will be appreciated that the task ofsplitting a main radiation beam B having a rectangular cross-sectioninto substantially similar branch radiation beams B_(a)-B_(c) issignificantly easier than splitting a main radiation beam B having anelliptical cross-section into substantially similar branch radiationbeams B_(a)-B_(c). For example, the modified rectangular main radiationbeam B which is output from the radiation alteration device 1014 maysimply be split into a plurality of substantially equal branch radiationbeams B_(a)-B_(c) by providing substantially rectangular shapedreflective facets 2007 a-2007 c in the path of the modified mainradiation beam B.

Modification of the main radiation beam B with a radiation alterationdevice prior to splitting the main radiation beam B into branchradiation beams B_(a)-B_(c) may also advantageously allow the mainradiation beam B to be split into branch radiation beams each havingsubstantially the same power. As was described above the radiationalteration device 1014 serves to increase the homogeneity of the mainradiation beam B such that the spatial intensity profile of the modifiedradiation beam which is output from the radiation alteration device 1014may be substantially homogenous. If the spatial intensity profile of themodified radiation beam which is output from the radiation alterationdevice 1014 is homogenous then branch radiation beams B_(a)-B_(c) havingapproximately equal powers may be provided simply be splitting the mainradiation beam B into branch radiation beams B_(a)-B_(c) havingapproximately equal cross-sections.

Providing a radiation alteration device 1014 before a beam splittingapparatus 2001 is further advantageous because the radiation alterationdevice 1014 decreases the sensitivity of the spatial intensity profileof the main radiation beam B to changes in the main beam pointingdirection (as was described above with reference to the embodimentdepicted in FIG. 4). The sensitivity of the power of each of the branchradiation beams B_(a)-B_(c) to changes in the pointing direction of themain radiation beam B is therefore advantageously decreased by providinga radiation alteration device 1014 upstream of the beam splittingapparatus 2001.

An alternative beam splitting apparatus which may be used in somelithographic systems comprises a diffraction grating configured to splita main radiation beam B into a plurality of diffraction orders whichform branch radiation beams. The power and position of each of thebranch radiation beams which are formed by a beam splitting apparatuscomprising a diffraction grating may be highly sensitive to changes inthe pointing direction of the main radiation beam B and to changes inthe wavelength of the main radiation beam B. A radiation beam which isemitted from a free electron laser FEL may experience variations in bothpointing direction and wavelength and thus branch radiation beams whichare formed by a beam splitting apparatus comprising a diffractiongrating may undergo significant variations in both power and position.By comparison, a branch radiation beams formed by a beam splittingapparatus which comprises a plurality of reflective facets whichreceives radiation output from a radiation alteration device asdescribed herein is advantageously relatively insensitive to variationsin both the wavelength and pointing direction of a main radiation beamB.

Whilst a specific example, of a beam splitting apparatus 2001 is shownin FIG. 13 comprising three reflective facets 2007 a-2007 c which splita main radiation beam B into three branch radiation beams B_(a)-B_(c)other arrangements of a beam splitting apparatus 2001 may instead beused which may comprise more or less than three reflective facets 2007a-2007 c. For example alternative arrangements of a beam splittingapparatus which may be arranged to receive a modified main radiationbeam B output from a radiation alteration device 1014 are shown in FIGS.14A and 14B.

FIG. 14A is a schematic illustration of an embodiment of a beamsplitting apparatus 2001 a comprising three reflective facets 2007 a,2007 b and 2007 c. The reflective facets 2007 a, 2007 b and 2007 c areeach arranged to receive different portions of a modified main radiationbeam B which is output from the second opening 1224 of the radiationalteration device 1014 and reflect the different portions in differentdirections so as to split the main radiation beam B into separate branchradiation beams.

FIG. 14B is a schematic illustration of an embodiment of a beamsplitting apparatus 2001 b comprising four reflective facets 2007 a,2007 b, 2007 c and 2007 d. The reflective facets 2007 a, 2007 b, 2007 cand 2007 d are each arranged to receive different portions of a modifiedmain radiation beam B which is output from the second opening 1224 ofthe radiation alteration device 1014 and reflect the different portionsin different directions so as to split the main radiation beam B intoseparate branch radiation beams.

Whilst a specific embodiment of a radiation alteration device 1014 hasbeen described with reference to modifying a main radiation beam B priorto providing the modified main radiation beam to a beam splittingapparatus 2001, other embodiments of a radiation alteration device mayinstead be used to modify a main radiation beam B. For example, aradiation alteration device which includes a bend may be used to modifya main radiation beam B prior to providing the modified main radiationbeam to a beam splitting apparatus 2001. Additionally or alternatively aradiation alteration device having a cross-sectional shape which isdifferent to the cross-sectional shape of the radiation alterationdevice 1014 shown in FIG. 13 may be used. For example, a radiationalteration device having a square cross-section may be used. In generalthe cross-sectional shape of a radiation alteration device may beconfigured to as to provide a modified main radiation beam B having adesired cross-section. The desired cross-section of a modified mainradiation beam B which is output from a radiation alteration device maydepend on the arrangement of reflective facets which form a beamsplitting apparatus 2001 arranged to receive the modified main radiationbeam B.

In some embodiments it may be desirable to provide a beam splittingapparatus which is configured to split the modified main radiation beamB into a relatively large number of branch radiation beams. For example,a beam splitting apparatus may be provided which is configured to splita main radiation beam B into approximately ten branch radiation beams soas to provide ten different lithographic apparatus with a branchradiation beam.

As was described above with reference to other embodiments of aradiation alteration device which are configured to receive a branchradiation beam B_(a), the degree of scrambling of a radiation beam whichis caused by a radiation alteration device depends on the number ofreflections which radiation undergoes at an internal surface of theradiation alteration device. The number of reflections which radiationundergoes at an internal surface of the radiation alteration device isapproximately proportional to θL/D where θ is the half divergence withwhich a radiation beam enters a radiation alteration device, L is thelength of the radiation alteration device and D is the diameter of atube which forms the radiation alteration device. In order to achieve adesired degree of spatial scrambling a radiation alteration device whichreceives a main radiation beam B may have a relatively small diameter.

An embodiment of a radiation alteration device may, for example, have asubstantially rectangular cross-section and may have a second openingfrom which a modified main radiation beam B is output having dimensionsof approximately 3 mm in the y-direction and approximately 10 mm in thex-direction (the x and y-directions being perpendicular to a z-directionin which the main radiation beam propagates). The divergence of the mainradiation beam B which is input to the radiation alteration device maybe such that radiation which is output from the second opening of theradiation alteration device has a half divergence of approximately 3milliradians in the x-direction and approximately 10 milliradians in they-direction.

In order to reduce the amount of radiation which is lost from the mainradiation beam B due to absorption at a beam splitting apparatus it isdesirable for the reflective facets of a beam splitting apparatus to bearranged such that radiation is incident on the reflective facets and isreflected from the reflective facets at relatively small grazing angles.For example, the reflective facets of a beam splitting apparatus may bearranged such that radiation is incident on the reflective facets with agrazing angle of approximately 1 degree. Since the grazing angles atwhich the branch radiation beams are reflected are relatively small, theangular separation between the branch radiation beams will also berelatively small. Branch radiation beams which have a relatively smallangular separation between them and which are diverging beams mayoverlap with each other as they diverge away from a beam splittingapparatus and cannot therefore be treated as separate branch radiationbeams.

In order to prevent branch radiation beams from overlapping with eachother as they propagate away from a beam splitting apparatus, themodified main radiation beam B which is output from a radiationalteration device may be imaged onto a beam splitting apparatus with amagnification factor so as to reduce its divergence at the beamsplitting apparatus.

FIG. 15 is a schematic illustration of an arrangement in which amodified main radiation beam B which is output from the second opening1224 of a radiation alteration device 1014 is focused onto a beamsplitting apparatus 2001. The modified main radiation beam B which isoutput from the radiation alteration device 1014 is incident on afocusing optic 2015.

The focusing optic 2015 is configured to form an image of an objectplane 2011 (in which the second opening 1224 of the radiation alterationdevice lies) in an image plane 2013. The image which is formed in theimage plane 2013 is a magnified image of the second opening 1224 of theradiation alteration device 1014. That is, the cross-sectional size ofthe main radiation beam B in the image plane 2013 is greater than thecross-sectional size of the main radiation beam B in the object plane2011 (i.e. at the second opening 1224 of the radiation alteration device1014). Magnification of the main radiation beam B from the object plane2011 to the image plane 2013 means that the divergence of the mainradiation beam B in the image plane 2013 is less than the divergence ofthe main radiation beam B in the object plane 2011 (due to conservationof the etendue of the main radiation beam B between the object plane2011 and the image plane 2013). The divergence of the main radiationbeam is reduced by the magnification factor with which the object plane2011 is magnified at the image plane 2013. The focusing optic 2015therefore serves to decrease the divergence of the main radiation beam Bin the image plane 2011 when compared to the divergence of the mainradiation beam at the second opening of the radiation alteration device2011.

A beam splitting apparatus 2001 comprising a plurality of reflectivefacets is situated downstream of the image plane 2013 such that themagnified main radiation beam which passes through the image plane 2013is incident on the beam splitting apparatus 2001. Since the focusingoptic 2015 serves to decrease the divergence of the main radiation beamB, the main radiation beam B which is incident on the beam splittingapparatus 2001 has a smaller divergence than the divergence of the mainradiation beam B which is output from the radiation alteration device.The reduced divergence of the main radiation beam B at the beamsplitting apparatus means that the branch radiation beams which areformed at the beam splitting apparatus 2001 have a reduced divergence.The smaller divergence of the branch radiation beams may prevent thebranch radiation beams from overlapping with each other as they divergeaway from the beam splitting apparatus 2001. Magnification of the mainradiation beam B from the object plane 2011 to the image plane 2013 maytherefore be used to prevent the branch radiation beams produced at thebeam splitting apparatus 2001 from overlapping with each other such thateach branch radiation beam may be treated as a separate radiation beamand may be provided to a separate lithographic apparatus.

An additional advantage of imaging the radiation which is output from asecond opening of a radiation alteration device onto a beam splittingapparatus with a magnification factor, is that the cross-sectional sizeof the radiation beam at the beam splitting apparatus is greater thanthe cross-sectional size of the radiation beam at the second opening ofthe radiation alteration device. Increasing the cross-sectional size ofthe radiation beam which is incident on the beam splitting apparatus mayallow the size of each of the reflective facets of the beam splittingapparatus to be increased. Reflective facets having relatively smalldimensions may be difficult to manufacture and thus allowing the size ofthe reflective facets to be increased advantageously makes thereflective facets easier to manufacture.

For ease of illustration the focusing optic 2015 is depicted in FIG. 15as a transmissive focusing optic. However in practice the focusing optic2015 may be implemented as a reflective optical element as will be wellunderstood by a person skilled in the art. In some embodiments themagnification of the main radiation beam B from an object plane to animage plane may be performed using a plurality of optical elements. Forexample, in some embodiments the main radiation beam B may be magnifiedfrom an object plane to an image plane using two mirrors which arearranged similarly to the arrangement of mirrors in a Wolter telescope.

As has been described above branch radiation beams which are formed by abeam splitting apparatus 2001 which receives a main radiation beam Boutput from a radiation alteration device 1014 have a positivedivergence. Each branch radiation beam is directed to a respectivelithographic apparatus. In some embodiments the distance between a beamsplitting apparatus 2001 and a lithographic apparatus may be relativelylarge such that each branch radiation beam propagates along a relativelylong optical path before being provided to a lithographic apparatus. Forexample, the optical path of a branch radiation beam between a beamsplitting apparatus and a lithographic apparatus may be several tens ofmeters long.

In order to avoid a large increase in the cross-section of a branchradiation beam B_(a) between a beam splitting apparatus 2001 and alithographic apparatus LA_(a), the divergence of a branch radiation beamB_(a) may be decreased at a location downstream of the beam splittingapparatus 2001. For example, a branch radiation beam B_(a) which isoutput from a beam splitting apparatus 2001 may be directed to beincident on one or more focusing optics which may be configured todecrease the divergence of the branch radiation beam B_(a). The one ormore focusing optics may, for example, focus the branch radiation beamB_(a) such that the branch radiation beam B_(a) is close to being acollimated radiation beam. Since the etendue of the main radiation beamB is increased by the radiation alteration device 1014, the branchradiation beam B_(a) cannot be perfectly collimated by the one or morefocusing optics, however the one or more focusing optics may focus thebranch radiation beam B_(a) such that is close to being collimated.

As was described above with reference to, for example, FIG. 4, prior tobeing provided to a lithographic apparatus LA_(a) the branch radiationbeam B_(a) may be focused by a focusing optic 107 such that it enters aradiation alteration device 101 with a desired divergence. The radiationalteration device 101 modifies the branch radiation beam B_(a) and themodified branch radiation beam B_(a) is focused (e.g. with a secondfocusing optic 109 and a third focusing optic 111) to an intermediatefocus IF before being provided to a lithographic apparatus LA_(a).

In some embodiments of a lithographic system LS a main radiation beam Bwhich is provided to a beam splitting apparatus may comprise acombination of radiation beams which are emitted from a plurality offree electron lasers FEL. FIG. 16 is a schematic illustration of anarrangement in which radiation which is emitted from a plurality of freeelectron lasers is combined to form a plurality of main radiation beams.A first free electron laser FEL₁ emits a first free electron laser beamB_(FEL1) and a second free electron laser FEL₂ emits a second freeelectron laser beam B_(FEL2). A first mirror 5003 is arranged in aportion of the optical path of the first free electron laser beamB_(FEL1) such that approximately half of the cross-section of the firstfree electron laser beam B_(FEL1) is directed to be incident on a secondmirror 5005. A third mirror 5007 is arranged in a portion of the opticalpath of the second free electron laser beam B_(FEL2) such thatapproximately half of the cross-section of the second free electronlaser beam B_(FEL2) is directed to be incident on a fourth mirror 5009.The second mirror 5005 is situated adjacent to the portion of the secondfree electron laser beam B_(FEL2) which is not incident on the thirdmirror 5007 and is orientated so as to combine the portion of the firstfree electron laser beam B_(FEL1) which is incident on the first mirror5003 with the portion of the second free electron laser beam B_(FEL2)which is not incident on the third mirror 5007 to form a first combinedradiation beam B₂₁. The fourth mirror 5009 is situated adjacent to theportion of the first free electron laser beam B_(FEL1) which is notincident on the first mirror 5003 and is orientated so as to combine theportion of the second free electron laser beam B_(FEL2) which isincident on the third mirror 5007 with the portion of the first freeelectron laser beam B_(FEL1) which is not incident on the first mirror5003 to form a second combined radiation beam B₁₂. The mirrors 5003,5005, 5007, 5009 therefore form a beam combination apparatus which isconfigured to combine EUV radiation emitted from the first free electronlaser FEL₁ with radiation emitted from the second free electron laserFEL₂ to form the combined radiation beams.

The first combined radiation beam B₁₂ and the second combined radiationbeam B₂₁ each comprise a combination of radiation which is emitted fromthe first free electron laser FEL₁ and radiation which is emitted fromthe second free electron laser FEL₂. The first and second combinedradiation beams may each be used as main radiation beams which areprovided to respective first and second beam splitting apparatus 5001,5002 which split the combined radiation beams into branch radiationbeams. The branch radiation beams may be provided to a plurality oflithographic apparatus.

Forming a main radiation beam from a combination of radiation which isemitted from a plurality of free electron lasers advantageously providesredundancy such that if one of the free electron lasers develops a faultor is taken offline (e.g. for maintenance), each lithographic apparatuscontinues to receive radiation which is emitted from one or more otherfree electron lasers. The arrangement of mirrors which is shown in FIG.16 results in a first combined free electron laser beam B₂₁ and a secondcombined free electron laser beam B₁₂ each having a cross-sectioncomprising a first half being formed from radiation emitted from one ofthe free electron lasers FEL_(n), FEL₂ and a second half being formedfrom radiation emitted from the other of the free electron lasersFEL_(n), FEL₂. In the event that one of the free electrons FEL_(n), FEL₂stops emitting radiation the cross-sections of each of the first andsecond combined free electron laser beams B₂₁, B₁₂ will therefore behalved. If the first and second combined free electron laser beams B₂₁,B₁₂ were to be provided directly to a beam splitting apparatus (i.e.without passing through a radiation alteration device), then a halvingof the cross-sections of the first and second combined free electronlaser beams B₂₁, B₁₂ may cause some of the reflective facets which formthe beam splitting apparatus to receive no radiation. Consequently someof the branch radiation beams may not be formed and some lithographicapparatus may no longer be provided with radiation.

In order to reduce the impact of one of the free electron lasers FEL₁,FEL₂ ceasing to emit radiation, each of the first and second combinedfree electron laser beams B₂₁, B₁₂ are directed into respective firstand second radiation alteration devices 5011, 5012. In order to ensurethat the first and second combined free electron laser beams B₂₁, B₁₂enter their respective radiation alteration devices 5011, 5012 having apositive divergence, focusing optics 5013, 5014 are arranged to receivethe first and second combined free electron laser beams B₂₁, B₁₂ andfocus the beams such that they have a positive divergence. For ease ofillustration the focusing optics 5013, 5014 are depicted in FIG. 16 asbeing transmissive focusing optics. However in practice the focusingoptics 5013, 5014 may be implemented as reflective optics as will bewell understood by a person skilled in the art.

FIG. 17 is a schematic illustration of a cross-section through theradiation alteration device 5011 at a first opening of the radiationalteration device which receives the second combined free electron laserbeam B₁₂ from the focusing optics 5013. The second combined freeelectron laser beam B₁₂ comprises a first portion 5021 formed fromradiation emitted from the first free electron laser FEL₁ and a secondportion formed from radiation emitted from the second free electronlaser FEL₂. As was described above with reference to other embodimentsof a radiation alteration device, the radiation alteration device 5011is configured to spatially scramble the second combined free electronlaser beam B₂₁ to form a scrambled combined radiation beam. The spatialscrambling which is caused by the radiation alteration device 5011 issuch that the spatially scrambled combined radiation beam includesoverlap between radiation emitted from the first free electron laserFEL₁ and radiation emitted from the second free electron laser FEL₂.

The radiation beams which are emitted from the radiation alterationdevices 5011, 5012 are provided to beam splitting apparatus 5001, 5002which split the radiation beams into branch radiation beams B_(a)-B_(f).Since the radiation beams which are output from each of the radiationalteration devices 5011, 5012 comprises a scrambled combination ofradiation emitted from the first free electron laser FEL₁ and radiationemitted from the second free electron laser FEL₂, in the event that oneof the free electron lasers FEL₁, FEL₂ stops emitting radiation thecross-sections of the radiation beams which are provided to the beamsplitting apparatus 5001. 5002 may remain relatively unchanged.Consequently each of the reflective facets which form the beam splittingapparatus 5001, 5002 continue to receive radiation and thus each of thebranch radiation beams B_(a)-B_(h) continue to be formed by the beamsplitting apparatus 5001, 5002. The power of each of the radiation beamswhich are output from each of the radiation alteration devices 5011,5012 will be reduced in the event that one of the free electron lasersFEL₁, FEL₂ stops emitting radiation and thus the power of each of thebranch radiation beams B_(a)-B_(h) will be reduced. Whilst the power ofeach of the radiation beams which are output from each of the radiationalteration devices 5011, 5012 will be reduced in the event that one ofthe free electron lasers FEL₁, FEL₂ stops emitting radiation, thespatial scrambling which is caused by the radiation alteration devices5011, 5012 may ensure that the spatial distribution of power in theradiation beams output from the radiation alteration devices 5011, 5012will remain relatively unchanged. That is, a variation in the power ofradiation emitted by one or more of the first and second free electronlasers FEL₁, FEL₂ may cause the total power of the radiation which isoutput from the radiation alteration devices 5011, 5012 to vary but maynot cause a substantial variation in the spatial distribution of powerin the radiation beam which exits the radiation alteration devices 5011,5012. The lithographic apparatus which receive the branch radiationbeams B_(a)-B_(h) from the beam splitting apparatus 5001, 5002 willtherefore continue to receive radiation and may therefore continue tooperate. In some embodiments, the mirrors 5003, 5005, 5007 and 5009which are arranged to form the first and second combined free electronlaser beams B₂₁, B₁₂, may be moved out of the optical paths of the firstand second free electron laser beams B_(FEL1), B_(FEL2) during normaloperation of the first and second free electron lasers FEL₁, FEL₂. Thatis, when both the first and second free electron laser FEL₁, FEL₂ areemitting radiation, the first free electron laser beam B_(FEL1) may beprovided to the first radiation alteration device 5011 and the firstbeam splitting apparatus 5001, and the second free electron laser beamB_(FEL2) may be provided to the second radiation alteration device 5012and the second beam splitting apparatus 5002. In the event that one ofthe free electron lasers FEL₁, FEL₂ is to be taken offline or stopsemitting radiation, then the mirrors 5003, 5005, 5007 and 5009 may bemoved into to the optical paths of the first and second free electronlaser beams B_(FEL1), B_(FEL2) as is shown in FIG. 16. Moving themirrors 5003, 5005, 5007 and 5009 into to the optical paths of the firstand second free electron laser beams B_(FEL1), B_(FEL2) ensures thateach radiation alteration device 5011, 5012 and thus each beam splittingapparatus 5001, 5002 continues to receive radiation.

As has been described above, in some embodiments (e.g. the embodimentsshown in FIGS. 13, 14A, 14B, 15 and 16) a radiation alteration devicemay be positioned upstream of a beam splitting apparatus so as toprovide the beam splitting apparatus with a modified main radiation beamB. The beam splitting apparatus may split the modified main radiationbeam B into a plurality of branch radiation beams, each branch radiationbeam being provided to a separate lithographic apparatus. In someembodiments further radiation alteration devices are arranged andconfigured to modify each branch radiation beam before the branchradiation beam is provided to the lithographic apparatus (as is shown,for example, in FIG. 4). In such embodiments each branch radiation beammay therefore pass via two radiation alteration devices before beingprovided to a lithographic apparatus.

In other embodiments a branch radiation beam may only pass via oneradiation alteration device before being provided to a lithographicapparatus. For example, in some embodiments the main radiation beam Bmay not be modified by a radiation alteration device prior to beingincident on a beam splitting apparatus and a radiation alteration devicemay only be provided in the path of each branch radiation beam.Alternatively, a radiation alteration device may be arranged to modifythe main radiation beam B before the main radiation beam B is incidenton the beam splitting apparatus but no radiation alteration devices maybe provided in the path of the branch radiation beams. However,providing a radiation alteration device in the path of a branchradiation beam advantageously allows the cross-sectional shape of thebranch radiation beam to be modified prior to the branch radiation beambeing provided to a lithographic apparatus. For example, thecross-sectional shape of a branch radiation beam may be modified by aradiation alteration device such that the cross-sectional shape of thebranch radiation beam approximately matches the shape of a field facetmirror 10 on which the branch radiation beam is incident.

The ability of a radiation alteration device to modify thecross-sectional shape of a branch radiation beam before the branchradiation beam is provided to a lithographic apparatus may allow newconfigurations of beam splitting apparatus to be used. For example,without the use of a radiation alteration device, a beam splittingapparatus may be configured so as to provide branch radiation beamshaving cross-sectional shapes which are suitable for providing to alithographic apparatus. However in embodiments in which thecross-sectional shapes of the branch radiation beams are modified usingradiation alteration devices the beam splitting apparatus may providebranch radiation beams having any cross-sectional shape. Thecross-sectional shapes of the branch radiation beams may then bemodified by the radiation alteration devices so as to provide eachlithographic apparatus with a branch radiation beam having a desiredcross-sectional shape.

FIG. 18 is schematic illustration of a first portion 3001 a of anembodiment of a beam splitting apparatus which is configured to split amain radiation beam B into a plurality of branch radiation beams, whereeach branch radiation beam is formed from a sector of the cross-sectionof the main radiation beam B. The first portion 3001 a comprises fourreflective facets 3007 a-3007 d arranged in a diamond formation. Thereflective facet 3007 a-3007 d are configured to receive differentsectors of the cross-section a main radiation beam B and reflect thedifferent sectors in different directions so as to split the mainradiation beam into a plurality of branch radiation beams.

FIG. 19 is a schematic illustration of an arrangement of the firstportion 3001 a of the beam splitting apparatus and a second portion 3001b of the beam splitting apparatus, both the first and second portionsbeing arranged in the path of a main radiation beam B. The cross sectionof the main radiation beam B is shown in a first plane 3011 locatedupstream of the beam splitting apparatus. In the first plane 3011 themain radiation beam B has a substantially circular cross-section. Thefirst portion 3001 a of the beam splitting apparatus is arranged suchthat approximately half of the circular cross-section of the mainradiation beam B is incident on the reflective facets 3007 a-3007 b ofthe first portion 3001 a of the beam splitting apparatus. The firstportion of the beam splitting apparatus therefore splits half of thecross-section of the main radiation beam B into four separate branchradiation beams B_(a)-B_(d). The branch radiation beams B_(a)-B_(d) areshown in FIG. 19 in a second plane 3013 located downstream of the firstportion 3001 a of the beam splitting apparatus. Also shown in the secondplane 3013 is the remaining half 3017 of the main radiation beam B whichis not incident on the first portion 3001 a of the beam splittingapparatus.

A second portion 3001 b of the beam splitting apparatus is locateddownstream of the second plane 3013 and downstream of the first portion3001 a of the beam splitting apparatus. Similarly to the first portion3001 a of the beam splitting apparatus, the second portion 3001 bcomprises four reflective facets arranged in a diamond formation. Thereflective facets of the second portion 3001 b of the beam splittingapparatus are arranged to receive the remaining half 3017 of the mainradiation beam B which was not split into branch radiation beamsB_(a)-B_(d) by the first portion 3001 a of the beam splitting apparatus.The reflective facets of the second portion 3001 b of the beam splittingapparatus split the main radiation beam B into four branch radiationbeams B_(e)-B_(h). The branch radiation beams B_(a)-B_(d) which areformed by the first portion 3001 a of the beam splitting apparatus andthe branch radiation beam B_(a)-B_(h) which are formed by the secondportion 3001 b of the beam splitting apparatus are shown in a thirdplane 3015 located downstream of the second portion 3001 b of the beamsplitting apparatus. It can be seen from FIG. 19 that the branchradiation beams B_(a)-B_(h) correspond to different segments of thecross-section of the main radiation beam B.

FIG. 20A is a schematic illustration of the reflective facets 3007a-3007 h which form the first portion 3001 a and the second portion 3001b of the beam splitting apparatus as viewed along the direction ofpropagation of the main radiation beam B. The position of the mainradiation beam B on the reflective facets 3007 a-3007 h is indicated inFIG. 20A with a dashed circle. In the example which is shown in FIG. 20Athe centre of the main beam B is aligned with an intersection 3019 ofthe reflective facets 3007 a-3007 h. The reflective facets 3007 a-3007 htherefore split the main beam B into branch radiation beams B_(a)-B_(h)which correspond to equal sized sectors of the cross-section of the mainradiation beam B.

The cross-sectional intensity profile of the main radiation beam B maynot be homogenous but may, for example, be rotationally symmetric. Forexample, the cross-sectional intensity profile of the main radiationbeam B may be a two-dimensional Gaussian distribution. If thecross-sectional intensity profile of the main radiation beam B isrotationally symmetric and the centre of the main beam B is aligned withthe intersection 3019 of the reflective facets 3007 a-3007 h then eachbranch radiation beam B_(a)-B_(h) corresponding to a sector of thecross-section of the main radiation beam B will have approximately thesame power.

FIG. 20B is a schematic illustration of the reflective facets 3007a-3007 h and the main radiation beam B in a case where the centre of themain radiation B is not aligned with the intersection 3019 of thereflective facets 3007 a-3007 h. It can be seen from FIG. 20B that whenthe centre of the main beam B is not aligned with the intersection 3019of the reflective facets 3007 a-3007 h, each facet receives adifferently sized portion of the main radiation beam B. Consequently thebranch radiation beams B_(a)-B_(h) correspond to differently sizedportions of the main radiation beam B and may therefore have differentpowers. In such an arrangement the powers of the branch radiation beamsB_(a)-B_(h) are therefore sensitive to variations in the pointingdirection of the main radiation beam B which will lead to variations inthe alignment of the main radiation beam B relative to the reflectivefacets 3007 a-3007 h.

In order to reduce the sensitivity of the powers of the branch radiationbeams B_(a)-B_(h) to variations in the pointing direction of the mainradiation beam B, branch radiation beams may be formed by combiningradiation which is reflected from radially opposite reflective facets.For example, radiation which is reflected from the reflective facet 3007a may be combined with radiation which is reflected from the reflectivefacet 3007 e to form a first branch radiation beam B₁, radiation whichis reflected from the reflective facet 3007 b may be combined withradiation which is reflected from the reflective facet 3007 f to form asecond branch radiation beam B₂, radiation which is reflected from thereflective facet 3007 c may be combined with radiation which isreflected from the reflective facet 3007 g to form a third branchradiation beam B₃ and radiation which is reflected from the reflectivefacet 3007 d may be combined with radiation which is reflected from thereflective facet 3007 h to form a fourth branch radiation beam B₄. Thecontribution of radiation which is reflected from each of the reflectivefacets to their respective branch radiation beams will therefore vary asa function of the pointing direction of the main radiation beam B butthe power of each of the first, second, third and fourth branchradiation beams will advantageously be relatively insensitive to thechanges in the pointing direction of the main radiation beam B.

FIG. 21 is a schematic illustration of an arrangement of mirrors whichmay be used to combine radiation which is reflected from differentreflective facets so as to form a single branch radiation beam B₁. Inthe example, which is shown in FIG. 21 a portion B′ of a main radiationbeam B is reflected from a first reflective facet 3007 a and a secondportion B″ of the main radiation beam B is reflected from a secondreflective facet 3007 e. The first reflective facet 3007 a and thesecond reflective facet 3007 e may be arranged so as to receive andreflect radially opposite sectors of the cross-section of the mainradiation beam B. A first mirror 3021 is arranged to reflect the firstportion B′ to be incident on a second mirror 3025. A third mirror 3023is arranged to reflect the second portion B″ such that the first portionB′ which is reflected from the second mirror 3025 and the second portionB″ which is reflected from the third mirror 3023 propagate insubstantially the same direction and adjacent to each other to form afirst branch radiation beam B₁. It will be appreciated that furthersimilar arrangements of mirrors may be used to combine other portions ofthe main radiation beam B which are reflected from other reflectivefacets so as to form further branch radiation beams.

In the embodiment of a beam splitting apparatus which is shown in FIGS.18 and 19 each of the reflective facets of the first portion 3001 a ofthe beam splitting apparatus intersect at a single intersection point3019. Similarly the reflective facets of the second portion 3001 b ofthe beam splitting apparatus also intersect at a single intersectionpoint. Each reflective facet will therefore have a sharp tip at theintersection point which may be difficult to manufacture.

FIG. 22A is a schematic illustration of an alternative arrangement of abeam splitting apparatus which allows the beam splitting apparatus to beformed from reflective facets which do not have sharp tips. The mainradiation beam B is incident on a first conical lens 3031 and a secondconical lens 3032. The first conical lens 3031 and the second conicallens 3032 are configured to direct the main radiation beam B to form anannular ring of radiation B_(r). An annular ring of radiation beam B_(r)may be conveniently split into branch radiation beams by reflectivefacets which do not comprise sharp tips. For example, reflective facets3007 a and 3007 e are shown in FIG. 22A splitting the annular ring ofradiation B, into branch radiation beams B_(a) and B_(e) respectively.For ease of illustration the conical lenses 3031, 3032 are depicted inFIG. 22A as being transmissive focusing optics. However in practice theconical lenses 3031, 3032 may be implemented as reflective optics aswill be well understood by a person skilled in the art.

FIG. 22B is a schematic illustration of the reflective facets 3007a-3007 h which are arranged to split an annular ring of radiation B_(r)into a plurality of branch radiation beams, as viewed along thedirection of propagation of the annular ring of radiation B_(r). It canbe clearly seen from FIG. 22B that the annular ring of radiation B_(r)may be split into branch radiation beams with reflective facets 3007a-3007 h which do not comprise sharp tips.

The use of conical lenses to expand a main radiation beam B into anannular ring of radiation B_(r) may enable further alternativearrangements of a beam splitting apparatus to be advantageously used.FIG. 23 is a schematic illustration of an alternative embodiment of abeam splitting apparatus 4001 which is configured to split an annularring of radiation B_(r) into a plurality of branch radiation beamsB_(a)-B_(h). The annular ring of radiation B_(r) which is shown in afirst plane located upstream of the beam splitting apparatus 4001 may,for example, be formed from a main radiation beam B by one or moreconical lenses (e.g. the first and second conical lenses shown in FIG.22A). The beam splitting device 4001 has a generally conical shape withan exterior surface which forms a plurality of reflective facets. Asingle one of the reflective facets is labeled 4007 a in FIG. 23. Thereflective facets are each configured to reflect a sector of the annularring of radiation B_(r) to form a plurality of branch radiation beamsB_(a)-B_(h). The branch radiation beams B_(a)-B_(h) are shown in asecond plane 4011 which is located downstream of the beam splittingapparatus 4001 in FIG. 23.

One or more of the branch radiation beams B_(a)-B_(h) which are shown inFIG. 23 may be combined with each other to form combined branchradiation beams. For example, an arrangement of mirrors similar to themirrors which are shown in FIG. 21 may be used to combine radiationwhich is reflected from radially opposite reflective facets to formbranch radiation beams whose power is relatively insensitive to changesin the pointing direction of the main radiation beam B.

Whilst the embodiment of a beam splitting apparatus 4001 which is shownin FIG. 23 has been described in connection with splitting up an annularring of radiation B_(r) into branch radiation beams, it will beappreciated that the beam splitting apparatus 4001 of FIG. 23 may beadapted such that it is suitable to split a main radiation beam B intobranch radiation beams B_(a)-B_(h) without first expanding the mainradiation beam B into an annular ring of radiation B_(r). For example,each of the reflective facets which form the beam splitting apparatuswhich is shown in FIG. 23 may be extended towards the first plane 4009in FIG. 23 such that the reflective facets intersect at a point. Such abeam splitting apparatus may be suitable for splitting a main radiationbeam B into branch radiation beams B_(a)-B_(h) without first expandingthe main radiation beam B into an annular ring of radiation B_(r).

Several embodiments of a beam splitting apparatus have been describedabove with reference to FIGS. 18-23 in which a main radiation beam B issplit into a plurality of branch radiation beams, each branch radiationbeam corresponding to one or more sectors of the cross-section of themain radiation beam B. It will be appreciated that the branch radiationbeams which are formed by such beam splitting apparatus havecross-sectional shapes which may not match the cross-sectional shape ofa field facet mirror 10 on which the branch radiation beams are directedto be incident. Each branch radiation beam may therefore be modified byone or more radiation alteration devices which may modify thecross-sectional shapes of the branch radiation beams such that thecross-sectional shapes of the branch radiation beams approximatelycorrespond to the cross-sectional shapes of field facet mirrors on whichthe branch radiation beams are incident.

Several embodiments of a beam splitting apparatus have been describedabove with reference to FIGS. 18-23 in which a main radiation beam B issplit into a plurality of branch radiation beams, each branch radiationbeam corresponding to one or more sectors of the cross-section of themain radiation beam B. The term “sector” is intended to mean a portionof the cross-section of the main radiation beam which is bound by tworadial lines both extending from a single point. A sector of thecross-section of a radiation beam may, for example, correspond to asector of a circular cross-section as is shown, for example, in FIG.20A. Alternatively a sector of the cross-section of a radiation beam maycorrespond to a sector of an annular ring of radiation as is shown inFIG. 22B.

Whilst the embodiments of the beam splitting apparatus which aredepicted in FIGS. 18-23 have been described with reference to a mainradiation beam B which has not passed through a radiation alterationdevice, in some embodiments a radiation alteration device may be locatedupstream of the beam splitting apparatus shown in FIGS. 18-23 such thatthe beam splitting apparatus receive a modified main radiation beam Bwhich has passed through a radiation alteration device.

Various embodiments of a radiation alteration device have been describedabove in which the radiation alteration device comprises a tube having areflective internal surface. In other embodiments, a radiationalteration device may comprise a plurality of reflective facets. FIG. 24is a schematic illustration of a radiation alteration device 6101 whichcomprises a plurality of reflective facets 6103 a-6103 d. The radiationalteration device 6101 receives a radiation beam 620. The radiation beam620 may, for example, be a branch radiation beam B_(a) received from abeam splitting apparatus. Alternatively the radiation beam 620 may be amain radiation beam B emitted from a free electron laser FEL.

Each of the reflective facets 6103 a-6103 d receive a portion of theradiation beam 620 and reflect the received portion so as to form aplurality of sub-beams 620 a-620 d. For example, a first reflectivefacet 6103 a reflects a portion of the radiation beam 620 to form afirst sub-beam 620 a. Each of the reflective facets 6103 a-6103 dcomprises a concave reflective surface and are configured to focus thesub-beams to a respective focal point 621 a-621 d. The focal points 621a-621 d each lie in a plane of focal points 6210. In some embodimentsthe focal points 621 a-621 d may be substantially uniformly spaced fromeach other in the plane of focal points 6210. In the embodiment which isshown in FIG. 24, the plane of focal points 6210 is approximatelyparallel with a plane 6310 in which the reflective facets 6103 a-6103 dgenerally lie. As is shown in FIG. 24 the reflective facets 6103 a-6103d each comprise a curved reflective surface. The reflective surfaces ofthe facets 6103 a-6103 d do not therefore lie exactly in the plane 6310.The plane 6310 merely represents a plane in which the reflective facets6103 a-6103 d generally lie.

As was described above with reference to other embodiments, theradiation beam 620 which is incident on a radiation alteration devicemay have a relatively low etendue. The reflective facets 6103 a-6103 dof the radiation alteration device 6101 are configured to focus anddirect the sub-beams 620 a-620 d such that the sub-beams 620 a-620 doverlap in a far field location so as to form a modified radiation beam.When viewed in a far field location, the plurality of focal points 621a-621 d act as a planar high-etendue light source. The modifiedradiation beam therefore has a significantly larger etendue than theradiation beam 620 which is incident on the radiation alteration device6101. The modified radiation beam may additionally have a spatialintensity profile in a far field which is significantly more homogeneousthan the spatial intensity profile of the radiation beam 620 which isincident on the radiation alteration device 6101. The reflective facets6103 a-6103 d may, for example, be configured to direct differentportions of the spatial intensity profile of the radiation beam 620which is incident on the radiation alteration device 6101 to differentlocations in a far-field plane so as to provide a modified radiationbeam having a substantially homogenous spatial intensity profile.

In the embodiment which is shown in FIG. 24 four reflective facets 6103a-6103 d are shown. In some embodiments a radiation alteration device6101 may comprise more than four reflective facets. FIG. 25 is aschematic illustration of a radiation alteration device 6101 whichcomprises sixteen reflective facets 6103 a-6103 p. The sixteenreflective facets 6103 a-6103 p which are shown in FIG. 25 may compriseall of the reflective facets of a radiation alteration device 6103 ormay comprise a portion of a radiation alteration device 6103. Forexample, a radiation alteration device 6103 may comprise more than thesixteen reflective facets 6103 a-6103 p that are shown in FIG. 25. In anembodiment, a radiation alteration device 6103 may comprise, forexample, approximately 44 reflective facets.

The reflective facets 6103 a-6103 p may be configured to cause themodified radiation beam which is formed by the radiation alterationdevice 6101 to have one or more desired properties. For example, thereflective facets 6103 a-6103 p may be configured to cause a desiredincrease in the etendue of the modified radiation beam when compared tothe radiation beam 620 which is incident on the radiation alterationdevice 6101.

In an embodiment the radiation beam 620 which is incident on theradiation alteration device 6101 may have a beam diameter ofapproximately 30 mm. The radiation beam 620 may be incident on theradiation alteration device at a grazing angle 6311 (as labeled in FIG.24) of approximately 8.5°. The radiation alteration device 6101 maycomprise 44 reflective facets so as to form 44 sub-beams. Each sub-beamwhich is incident on the radiation alteration device may be equivalentto a portion of the cross-section of the radiation beam 620. Eachportion of the cross-section of the radiation beam 620 which forms asub-beam at the radiation alteration device 6101 may have asubstantially square cross-sectional shape in a plane which liesperpendicular to the direction of propagation of the radiation beam 620.The square-shaped cross-sections of each sub-beam may have dimensions ofapproximately 4 mm by 4 mm. The sub-beams are focused by the reflectivefacets to a plurality of focal points. The sub-beams may be focused suchthat the half divergence of each sub-beam downstream of its focal pointis approximately 7 milliradians. The focal lengths of the reflectivefacets may be approximately 285 mm. A length 6312 (as labeled in FIG.24) of the radiation alteration device 6101 may be approximately 200 mm.It may be desirable for the focal lengths of the reflective facets to begreater than the length of a focal plane (i.e. the length of the planeof focal points 6210). The length of the focal plane is approximatelyequal to the length 6312 of the radiation alteration device.

The sub-beams overlap with each other in a far field location so as toform a modified radiation beam. In the above described embodiment, themodified radiation beam has a half divergence of approximately 7milliradians (corresponding to the half divergence of each sub-beam) andan apparent source size having a diameter of approximately 30 mm(corresponding to the diameter of the radiation beam 620 which isincident on the radiation alteration device). The etendue of themodified radiation beam is of the order of the square of the product ofthe diameter of the apparent source size and the half divergence of themodified radiation beam. As was described above, the modified radiationbeam which is formed by the radiation alteration device 6101 has asignificantly higher etendue than the radiation beam 620 which isincident on the radiation alteration device 6101.

The cross-sectional shape of the modified radiation beam which is formedby a radiation alteration device in a far field location, depends atleast in part on the cross-sectional shapes of the reflective facets.For example, the reflective facets 6103 a-6103 p which are shown in FIG.25 each have approximately rectangular cross-sections. The modifiedradiation beam which is formed by the reflective facets 6103 a-6103 p ina far field location may have an approximately square cross-sectionalshape. In some embodiments, the modified radiation beam which is formedby the reflective facets 6103 a-6103 p may have an approximatelyrectangular cross-sectional shape.

It may be desirable to form a modified radiation beam having across-sectional shape which approximately matches the shape of anoptical element which receives the modified radiation beam. For example,in some embodiments a modified radiation beam may be provided to alithographic apparatus and may be incident on a field facet mirror inthe lithographic apparatus. In such embodiments, a mismatch between thecross-sectional shapes of the modified radiation beam and a field facetmirror may result in radiation being lost from the modified radiationbeam at the field facet mirror. It may therefore be desirable to providea modified radiation beam having a cross-sectional shape whichapproximately matches the cross-sectional shape of the field facetmirror.

In some embodiments the reflective facets which form a radiationalteration device may have cross-sectional shapes which are configuredto form a modified radiation beam having a desired cross-sectionalshape. FIG. 26 is a schematic illustration of an embodiment of aradiation alteration device 6101′ comprising a plurality of reflectivefacets 6103′a-6103′m. The reflective facets 6103′a-6103′m each have anapproximately hexagonal cross-section and are arranged in the form of ahoneycomb lattice. The reflective facets 6103′a-6103′m may be configuredto form a modified radiation beam in a far field location which has anapproximately hexagonal cross-sectional shape. The honeycomb latticewhich is shown in FIG. 26 comprises a plurality of reflective facets6103′a-6103′m each having a stretched hexagonal shape such that thecross-sectional shape of a portion of the radiation beam which isincident on a single facet 6103′a-6103′m is approximately a regularhexagon.

In some embodiments, a modified radiation beam which is formed by theradiation alteration device 6101′ may be incident on a field facetmirror having an approximately circular cross-sectional shape. Thehexagonal cross-section of a modified radiation beam which is formed bythe radiation alteration device 6101′ may be approximately matched tothe circular shape of the field facet mirror so as to reduce any loss ofradiation from the modified radiation beam at the field facet mirror.

In the embodiment of a radiation alteration device 6101 which is shownin FIG. 24, the reflective facets 61033 a-6103 d are arranged to focussub-beams 620 a-620 d to focal points 621 a-621 d which lie in a planeof focal points 6210 which is approximately parallel with a plane 6310in which the reflective facets 6103 a-6103 d generally lie. In otherembodiments the plane of focal points 6210 may not be parallel with aplane 6310 in which the reflective facets 6103 a-6103 d generally lie.

FIG. 27 is a schematic illustration of an embodiment of a radiationalteration device 7101. The radiation alteration device 7101 comprisesreflective facets 7103 a-7103 c which lie generally in a plane 7310. Aradiation beam 720 is incident on the radiation alteration device 7101and is focused into a plurality of sub-beams 720 a-720 c by thereflective facets 7103 a-7103 c. The sub-beams 720 a-720 c are focusedto a plurality of focal points 721 a-721 c which each lie in a plane offocal points 7210. In the embodiment of FIG. 27 the reflective facets7103 a-7103 c are arranged such that the plane of focal points 7210 isnot parallel with the plane 7310 in which the reflective elements 7103a-7103 c generally lie.

Each of the sub-beams 720 a-720 c has a central optical axis 723 a-723c. In the embodiment of FIG. 27, the reflective facets 7103 a-7130 c arearranged such the plane of focal points 7210 is substantiallyperpendicular to the optical axes 723 a-723 c of the sub-beams 720 a-720c.

In embodiments in which the plane of focal points 7210 is not parallelwith a plane 7310 in which the reflective facets 7103 a-7103 c generallylie (as is shown, for example, in FIG. 27), the size and/or the shape ofthe reflective facets may be different for different reflective facets.FIG. 28 is a schematic illustration of an embodiment of a radiationalteration device 7101′ as viewed from above. The radiation alterationdevice 7101′ comprises a plurality of reflective facets 7103′ which arearranged to focus a plurality of sub-beams to focal points which lie ina plane of focal points, where the plane of focal points issubstantially perpendicular to the optical axes of the sub-beams (as isshown, for example, in FIG. 27). As can be seen in FIG. 28 thereflective facets 7103′ have different sizes and shapes.

Different reflective facets having different sizes results in thecross-sectional sizes of the different portions of the radiation beam720 which form the sub-beams 720 a-720 c being different for each other.Consequently different sub-beams may have different powers. For example,a reflective facet having a relatively large cross-section may receiveand focus more radiation into a sub-beam than a reflective facet havinga relatively small cross-section. Consequently a sub-beam which isreflected from a reflective facet having a relatively largecross-section may have a higher power than a sub-beam which is reflectedfrom a reflective facet having a relatively small cross-section. Afurther consequence of differently sized and/or shaped reflective facetsmay result in the focal points 721 a-721 c being unevenly spaced apart(as can be seen from FIG. 27).

FIG. 29 is a schematic illustration of an alternative embodiment of aradiation alteration device 6151 comprising a plurality of reflectivefacets 6153 a-6153 p. The radiation alteration device 6151 is arrangedto receive a radiation beam 6120, such that the radiation beam 6120illuminates at least a portion 6155 of the radiation alteration device6151. Each reflective facet 6153 a-6153 p receives and reflects aportion of the radiation beam 6120 so as to form a plurality ofsub-beams (not shown in FIG. 29). In the embodiment which is shown inFIG. 29, the reflective facets 6153 a-6153 p are substantially flat suchthat they do not have any focusing power. In other embodiments thereflective facets 6153 a-6153 p may have a concave shape and have apositive focusing power. Alternatively the reflective facets 6153 a-6153p may have a convex shape and have a negative focusing power.

The reflective facets 6153 a-6153 p are configured such that thesub-beams which are formed at the reflective facets 6153 a-6153 poverlap with each other in a far field plane so as to form a modifiedradiation beam in the far field plane. The reflective facets may, forexample, be configured such that at least some of the sub-beamsilluminate substantially the same area in the far field plane. That is,at least some of the sub-beams may completely overlap with each other inthe far field plane. In some embodiments not all of the sub-beams formedat each of the reflective facets will completely overlap with each otherin a far field plane. In general, the reflective facets are arrangedsuch that if each reflective facet is fully illuminated with radiationthen each sub-beam will at least partially overlap with at least oneother sub-beam in a far field plane.

In an embodiment, the radiation alteration device 6151 may supply themodified radiation beam to a lithographic apparatus. In such anembodiment the sub-beams which are formed at the reflective facets 6153a-6153 p may, for example, overlap with each other in a plane in which afield facet mirror of the lithographic apparatus is situated. In suchembodiments one or more focusing optics may be positioned in between theradiation alteration device and the lithographic apparatus. For examplefocusing optics may be arranged to focus the modified radiation beam toan intermediate focus prior to being provided to a lithographicapparatus.

In an alternative embodiment, the modified radiation beam, formed at theradiation alteration device 6151, may be provided to a beam splittingapparatus (such as, for example, a beam splitting apparatus of the formshown in any of FIG. 13, 14A, 14B, 18, 19, 21, 22A, or 23). In such anembodiment the sub-beams which are formed at the reflective facets 6153a-6153 p may overlap with each other in a plane which is close to thebeam splitting apparatus.

As was mentioned above, the reflective facets 6153 a-6153 p areconfigured such that regions of a far field plane are illuminated withmore than one overlapping sub-beam. The intensity of radiation which isincident on a particular location in the far field plane is thereforedependent on the intensity of radiation which is reflected from morethan one of the reflective facets 6153 a-6153 p. Consequently, thedependence of the cross-sectional intensity profile in the far fieldplane on the cross-sectional intensity profile at the reflective facets6153 a-6153 p is reduced. In this way the radiation alteration device6151 serves to increase the homogeneity of the cross-sectional intensityprofile of the modified radiation beam. In some embodiments, if all ofthe reflective facets 6153 a-6153 p are completely illuminated then thereflective facets 6153 a-6153 p may form sub-beams which substantiallycompletely overlap with each other in the far field plane. In suchembodiments the intensity profile in the far field plane may besubstantially homogenous and may not strongly depend on the intensityprofile of the radiation beam 6120 incident on the reflective facets6153 a-6153 p.

By forming a modified radiation beam from sub-beams which overlap in afar field plane as was described above, the pointing direction and/orthe intensity profile of the modified radiation beam may be relativelyinsensitive to changes in the pointing direction of the radiation beam6120 which is incident on the radiation alteration device 6151. As wasexplained above, in some embodiments the reflective facets 6153 a-6165 pmay be configured such that if all of the reflective facets arecompletely illuminated then the reflective facets 6153 a-6153 p formsub-beams which substantially completely overlap with each other in thefar field plane. As is shown for example in FIG. 29, in practice theradiation beam 6120 may not completely illuminate all of the reflectivefacets 6153 a-6153 p. In such an embodiment, a change in the pointingdirection of the radiation beam 6120 may cause more radiation to beincident on some of the reflective facets 6153 a-6153 p and lessradiation to be incident on others of the reflective facets 6153 a-6153p. In an embodiment in which the sub-beams formed at the reflectivefacets overlap in a far field plane, a shift of radiation to differentreflective facets may cause little or no change in the pointingdirection and/or the intensity profile of the modified radiation beam inthe far field plane. The radiation alteration device 6151 thereforeadvantageously reduces the dependence of the pointing direction and/orthe intensity profile of the modified radiation beam in the far fieldplane, on the pointing direction of the radiation beam 6120 which isincident on the radiation alteration device 6151.

In the embodiment which is shown in FIG. 29 the radiation alterationdevice 6151 comprises a total of 16 reflective facets 6153 a-6153 p. Inother embodiments a radiation alteration device 6151 of the type shownin FIG. 29 may comprise more or fewer than 16 reflective facets. Forexample, in some embodiments a radiation alteration device may comprise8 rows and 8 columns of reflective facets (as opposed to the 4 rows and4 columns shown in FIG. 29) such that there is provided a total of 64reflective facets. In other embodiments of a radiation alterationdevice, a number of rows of reflective facets may be different to anumber of columns of reflective facets.

Increasing the number of reflective facets from which a radiationalteration device is formed may advantageously increase the homogeneityof the intensity profile of the modified radiation beam in the far fieldplane. As was described above, arranging the reflective facets such thatthey form overlapping sub-beams in the far field plane reduces thedependence of the intensity profile in the far field plane on anydifferences between the intensity of radiation incident on differentreflective facets. However, the intensity profile in the far field planemay still be sensitive to any inhomogeneities in the intensity profileof radiation which is incident on a single reflective facet. Forexample, if the intensity of radiation which is incident on a singlereflective facet includes spatial inhomogeneities then these spatialinhomogeneities may also be present in the intensity profile in the farfield plane.

Increasing the number of reflective facets from which a radiationalteration device is formed will usually result in a reduction in thesize of each reflective facet. Reducing the size of a reflective facetwill reduce the cross-section of a portion of the radiation beam 6120which is incident on a single reflective facet. Reducing thecross-section of a portion of the radiation beam 6120 which is incidenton a single reflective facet will typically lead to a reduction in thesize of any spatial inhomogeneities which are present in the intensityprofile of radiation which is incident on the single reflective facet.Consequently the size of any spatial inhomogeneities present in theintensity profile in the far field plane may be reduced.

Increasing the number of reflective facets from which a radiationalteration device is formed may additionally or alternatively reduce thedependence of the radiation in the far field plane on the pointingdirection of the radiation beam 6120 which is incident on the radiationalteration device 6151.

Increasing the number of reflective facets from which a radiationalteration device is formed may, however, increase the complexity and/orthe expense involved with manufacturing a radiation alteration device.The number of reflective facets from which a radiation alteration deviceis formed and the size of the reflective facets may be selected so as tofind a balance between the cost and complexity of manufacture and theperformance of the radiation alteration device.

The cross-sectional shapes of the reflective facets 6153 a-6153 p andthe focusing power of the reflective facets may, at least in part,determine the size and shape of the cross-section of the modifiedradiation beam in the far field plane. The cross-sectional shapes of thereflective facets 6153 a-6153 p may be selected such that thecross-section of the modified radiation beam in the far field planeapproximately matches one or more optical elements which are situated inor close to the far field plane. For example, the cross-section of themodified radiation beam in a far field plane (which may be located afterone or more focusing optics) may approximately match the cross-sectionof a field facet mirror which forms part of a lithographic apparatus.Alternatively the cross-section of the modified radiation beam in thefar field plane may approximately match the cross-section of a beamsplitting apparatus, on which the modified radiation beam is incident.

In the embodiment which is shown in FIG. 29 the reflective facets 6153a-6153 p each have approximately rectangular cross-sectional shapes. Asshown in FIG. 29, the radiation beam 6120 may be incident on theradiation alteration device 6151 at a grazing incidence angle such thatthe reflective facets receive approximately square-shapedcross-sectional portions of the radiation beam 6120. In otherembodiments, the reflective facets may have different cross-sectionalshapes than the shapes shown in FIG. 29 and/or the reflective facets maybe arranged differently to the arrangement shown in FIG. 29.

The radiation alteration device 6151 which is shown in FIG. 29 isdifferent to the radiation alteration devices which are shown in FIGS.24-28. In particular, the radiation alteration devices which are shownin FIGS. 24-28 each comprise a plurality of reflective facets configuredto direct and focus a plurality of sub-beams to a plurality of focalpoints which lie in a plane of focal points. As was explained above withreference to FIG. 24, when viewed in a far field plane, the plurality offocal points act as a planar high-etendue light source. In theembodiment which is shown in FIG. 29, a plurality of reflective facetsare not configured to focus the sub-beams to a plurality of focal pointswhich lie in a plane of focal points but are instead arranged to formsub-beams which overlap with each other in a far field plane. Both formsof radiation alteration device serve to provide a modified radiationbeam having an etendue which is greater than the etendue of a radiationbeam which is incident on the radiation alteration device. Both forms ofradiation alteration device also serve to provide a modified radiationbeam whose cross-sectional intensity profile is more homogenous than thecross-sectional intensity profile of a radiation beam which is incidenton the radiation alteration device. Both forms of radiation alterationdevice may provide a modified radiation beam in a far field plane whichis suitable for providing to a lithographic apparatus and/or a beamsplitting apparatus.

A radiation alteration device of the type shown in FIG. 29 may provide amodified radiation beam which is suitable for providing to alithographic apparatus and/or a beam splitting apparatus in a far fieldplane which lies relatively close to the radiation alteration device. Aradiation alteration device of the type shown in FIGS. 24-28 may providea modified radiation beam which is suitable for providing to alithographic apparatus and/or a beam splitting apparatus in a far fieldplane which lies relatively far away from the radiation alterationdevice. Either form of radiation alteration device may be used inconjunction with one or more focusing optics. For example, a modifiedradiation beam provided by a radiation alteration device may be focusedonto a far field plane by one or more focusing optics.

FIG. 30 is a schematic illustration of a portion of a lithographicsystem LS which includes a radiation alteration device of the type shownin FIG. 29. The lithographic system LS includes a radiation source SOwhich provides a radiation beam B. The radiation source SO may, forexample, include one or more free electron lasers FEL. The radiationbeam B is initially incident on beam expanding optics 7001. The beamexpanding optics 7001 expand the radiation beam B which is subsequentlyincident on a radiation alteration device 7100. The radiation alterationdevice 7100 provides a modified radiation beam 7003 which is incident ona beam splitting apparatus 7005. The beam splitting apparatus 7005splits the modified radiation beam 7003 into a plurality of branchradiation beams B_(a)-B_(c). In the example which is shown in FIG. 30the beam splitting apparatus 7005 splits the modified radiation beam7003 into three branch radiation beams B_(a)-B_(c). In some embodimentsthe beam splitting apparatus 7005 may split the modified radiation beam7003 into more or fewer than three branch radiation beams B_(a)-B_(c).

A first branch radiation beam B_(a) is incident on directing optics7006. The directing optics 7006 provide the first branch radiation beamB_(a) to a first focusing optic 7007 a and a second focusing optic 7007b. The first and second focusing optics 7007 a, 7007 b together focusthe modified radiation beam to an intermediate focus IF. Theintermediate focus IF is situated substantially at an opening in anenclosing structure of a lithographic apparatus LA_(a). The modifiedradiation beam passes through the opening in the enclosing structure andis incident on optical components (not shown) which form thelithographic apparatus LA_(a). For example, the modified radiation beammay initially be incident on a field facet mirror (not shown) whichforms part of the lithographic apparatus LA_(a).

Second B_(b) and third B_(c) branch radiation beams may also be providedto respective lithographic apparatuses (not shown) via respectivedirecting optics (not shown) and focusing optics (not shown). For easeof illustration, the optical path of the second B_(b) and third B_(c)branch radiation beams are omitted from FIG. 30.

For ease of illustration the beam expanding optics 7001 and theradiation alteration device 7100 are shown in FIG. 30 as being formedfrom transmissive optical components. However, in practice the beamexpanding optics 7001 and the radiation alteration device 7100 may beformed from reflective optics.

FIG. 31 is a schematic illustration of an example of beam expandingoptics 7001. In the example shown in FIG. 31, the beam expanding optics7001 comprises a first reflective element 7001 a and a second reflectiveelement 7001 b. The first reflective element 7001 a comprises a convexreflective surface which receives and reflects the radiation beam B. Thesecond reflective surface comprises a concave reflective surface whichreceives and reflects the radiation beam B. Together, the first andsecond reflective elements 7001 a, 7001 b serve to expand thecross-section of the radiation beam B. The cross-section of theradiation beam B may, for example, be expanded such that the radiationbeam B illuminates a majority of the radiation alteration device 7100.

In other embodiments the beam expanding optics may comprise any opticalcomponent or combination of optical components which serve to expand thecross-section of the radiation beam B. In some embodiments, the beamexpanding optics may comprise more than two optical components. Forexample the beam expanding optics may comprise four or more reflectiveelements. The reflective elements may, for example, include reflectivesurfaces whose shape conforms to an extruded parabola.

The radiation alteration device 7100 comprises a plurality of reflectivefacets which each receive a portion of the expanded radiation beam B.Each reflective facet reflects a received portion of the expandedradiation beam B so as to form a sub-beam. The reflective facets areconfigured such that the sub-beams overlap with each other in a farfield plane. The far field plane lies at or close to the beam splittingapparatus 7005. The radiation alteration device 7100 may, for example,be of the form described above with reference to FIG. 29.

The beam splitting apparatus 7005 may comprise a plurality of reflectivefacets each arranged to receive a portion of a modified radiation beamwhich is formed by the radiation alteration device 7100. The reflectivefacets may be arranged to reflect respective portions of the modifiedradiation beam in different directions so as to form branch radiationbeams B_(a)-B_(c). The beam splitting apparatus 7005 may be of the formshown in any of FIG. 13, 14A, 14B, 18, 19, 21, 22A, or 23 or may takeany other form.

The directing optics 7006 receive the first branch radiation beam B_(a)from the beam splitting apparatus 7005 direct the branch radiation beamB_(a) to the focusing optics 7007 a, 7007 b. FIG. 32 is a schematicrepresentation of optical elements which form an embodiment of thedirecting optics 7006. In the embodiment which is shown in FIG. 32, thedirecting optics are formed from four optical elements 7006 a-7006 d.Each optical element forms an image of the branch radiation beam B_(a).In the representation which is shown in FIG. 32, the optical elementsare depicted as transmissive optics. The transmissive optics are shownon a scale which extends in y and z-directions. The dimensions in they-direction are given in millimeters and the dimensions in thez-direction are given in meters. Each optical element 7006 a-7006 d hasits own focal length f. The focal lengths f of each of the opticalelements 7006 a-7006 d are labelled in FIG. 32.

Whilst the optical elements 7006 a-7006 d which are shown in FIG. 32 aredepicted as transmissive optics in FIG. 32, in practice each opticalelement 7006 a-7006 d may be formed from one or more reflectiveelements. For example, each optical element 7006 a-7006 d may comprise aplurality of grazing incidence mirrors. In some embodiments one or moreof the optical elements may comprise back to back Wolter telescopes.

In the example which is shown in FIG. 32 the branch radiation beam has ahalf divergence θ of approximately 4 miliradians. The branch radiationbeam may initially have a beam diameter of approximately 10 mm. Theoptical elements 7006 a-7006 d are configured such that the diameter ofthe branch radiation beam B_(a) does not exceed approximately 40 mm.

Delivering a radiation beam over a given distance using delivery opticssuch as those shown in FIG. 32, may be simplified if the etendue of theradiation beam is reduced. In the embodiment which is shown in FIG. 30,the etendue of the branch radiation beam is increased by the radiationalteration device 7100 which comprises a plurality of reflective facets.In other embodiments the etendue of a branch radiation beam may beincreased by a radiation alteration device comprising a tube having areflective internal surface (as is shown for example in FIG. 13). Aradiation alteration device comprising a tube having a reflectiveinternal surface may typically increase the etendue of a radiation beamto a greater extent than an equivalent radiation alteration devicecomprising a plurality of reflective facets. The use of a radiationalteration device comprising a plurality of reflective facets prior toproviding a radiation beam to a beam splitting apparatus (as is shown inFIG. 30) may therefore simplify the directing optics 7006 used to directa branch radiation beam B_(a) to a lithographic apparatus (when comparedto using a radiation alteration device comprising a tube having areflective internal surface).

As was previously explained, the etendue of a modified radiation beam isof the order of the square of the product of the diameter of theapparent source size and the half divergence of a modified radiationbeam. In the example which is shown in FIG. 32, the diameter of theapparent source size of the branch radiation beam which is provided tothe directing optics may be approximately 10 mm and the half divergencemay be approximately 4 miliradians. The product of the diameter of theapparent source size and the half divergence is therefore approximately40 mm miliradians. In an alternative embodiment in which the radiationalteration device comprises a tube having a reflective internal surface,the equivalent diameter of the apparent source size of the branchradiation beam may be approximately 5 mm and the equivalent halfdivergence may be approximately 20 miliradians. The product of thediameter of the apparent source size and the half divergence istherefore approximately 100 mm miliradians. The etendue of the branchradiation beam may therefore be significantly smaller when a radiationalteration device comprising a plurality of reflective facets is usedprior to the beam splitting apparatus (when compared to a radiationalteration device comprising a tube having a reflective internalsurface). As was explained above this may simplify the design of thedirecting optics which direct the branch radiation beam B_(a) to alithographic apparatus.

FIG. 33 is a schematic illustration of a portion of the beam splittingapparatus 7005. As was described above the beam splitting apparatus 7005may, for example, be of the form shown in any of FIG. 13, 14A, 14B, 18,19, 21, 22A, or 23 or may take any other form. FIG. 33 shows a singlereflective facet 7005 a which forms part of the beam splitting apparatus7005. The reflective facet 7005 a has a height H. The reflective facet7005 a also has a width W (not shown in FIG. 33) which extends into thepage of FIG. 33. A modified radiation beam is incident on the reflectivefacet 7005 a having an angular range of propagation directions 2θ. Thatis, the angular range of propagation directions is two times thehalf-divergence θ of the modified radiation beam. The average grazingincidence angle with which the modified radiation beam is incident onthe reflective facet 7005 a is labelled φ in FIG. 33.

As was described above with reference to FIG. 30 the beam splittingapparatus is positioned at or near a far field plane in which sub-beamsformed at the radiation alteration device 7100 overlap with each other.In particular, it may be desirable for the entire surface of thereflective facets of the beam splitting apparatus to be close to the farfield plane in which the sub-beams overlap. The entire beam splittingapparatus may be considered to be close to the far field plane if thefollowing inequality is satisfied.

$\begin{matrix}{\frac{{HD}_{0}}{W\;\varphi\; L} ⪡ 1} & (3)\end{matrix}$Where D_(o) is the diameter of the radiation beam which is incident onthe radiation alteration device 7100 and N is the number of reflectivefacets of the beam splitting apparatus and thus the number of branchradiation beams to be formed.

It may be further desirable for substantially all rays of radiationwhich form the modified radiation beam to be incident on one of thereflective facets which form the beam splitting apparatus. This may beachieved if the following inequality is satisfied.

$\begin{matrix}{\frac{D_{0}}{\;{\varphi\; L}} ⪡ 1} & (4)\end{matrix}$Where L is the distance between the radiation alteration device 7100 andthe beam splitting apparatus 7005.

It may be further desirable for the facets of the beam splittingapparatus to be sufficiently large such that the branch radiation beamsare separated by an angle which is larger than the angular spread 2θ.This may be achieved if the following inequality is satisfied.

$\begin{matrix}{\frac{D_{0}N}{\;{2{\pi\varphi}\; L}} < 1} & (5)\end{matrix}$

In an embodiment the variables mentioned above may take on approximatelythe following values L=4 m, D₀=20 mm, W=100 mm, H=10 mm, N=10, φ=50miliradians. In such an embodiment the inequalities given in equations(3)-(5) are all satisfied.

FIG. 34 is a schematic illustration of an alternative embodiment of aradiation alteration device 8001 comprising a first diffusing element8003 a and a second diffusing element 8003 b. The first diffusingelement 8003 a comprises a first roughened reflective surface 8005 a andthe second diffusing element 8003 b comprises a second roughenedreflective surface 8005 b. In the embodiment which is shown in FIG. 34the first and second diffusing elements 8003 a, 8003 b both comprisedisc like structures which include roughened reflective surfaces 8005 a,8005 b.

A radiation beam 8020 is incident on the first roughened reflectivesurface 8005 a and reflected from the first roughened reflective surface8005 a so as to be incident on the second roughened reflective surface8005 b. The radiation beam is reflected from the second roughenedreflective surface 8005 b so as to output a modified radiation beam8007.

FIG. 35 is a schematic illustration of a portion of the first roughenedreflective surface 8005 a. The roughened reflective surface 8005 aincludes indentations 8009 which cause the reflective surface 8005 a todeviate from a flat plane 8010 which is shown in FIG. 35 for referencepurposes. Also shown in FIG. 35 are rays of radiation 8012 whichrepresent parts of the radiation beam 8020 and which are incident on theroughened reflective surface 8005 a. The radiation beam 8020 and therays 8012 are incident on the roughened reflective surface 8005 a at agrazing incidence angle φ relative to the flat plane 8010 from which theroughened reflective surface 8005 a deviates.

Due to the deviations of the roughened surface 8005 a from the flatplane 8010 different rays of radiation 8012 will be incident on thereflective surface 8005 a at different grazing incidence angles relativeto the roughened reflective surface 8005 a. Consequently different raysof radiation 8012 will be reflected from the roughened reflectivesurface in different directions. The roughened reflective surface 8005 atherefore serves to increase a range of angles with which rays ofradiation 8012, which form the radiation beam 8012, propagate. That is,the roughened reflective surface 8005 a introduces an angular spread tothe radiation beam 8012.

Most of the angular spread which is introduced into the radiation beam8020 is in a direction which is approximately parallel to the plane ofincidence at the roughened reflective surface 8005 a. The secondroughened reflective surface 8005 b of the second diffusing element 8003b may be similar to the first roughened reflective surface 8005 a whichis shown in FIG. 35. The second roughened reflective surface 8005 b maytherefore also introduce an angular spread into the radiation beam 8020.The second roughened reflective surface 8005 b may be arranged such thatit is approximately perpendicular to the first roughened reflectivesurface 8005 a. The plane of incidence at the second roughenedreflective surface 8005 b may therefore be approximately perpendicularto the plane of incidence at the first roughened reflective surface 8005a. The angular spread which is introduced at the second roughenedsurface 8005 b may therefore generally be in a direction which isapproximately perpendicular to the general direction in which an angularspread is introduced at the second roughened surface 8005 b. Themodified radiation beam 8007 which is output from the radiationalteration device 8001 may therefore include angular spread in alldirections. This is illustrated in FIG. 34 by rays of radiation 8012which propagate in a range of different directions.

The angular spread of radiation which is introduced by the radiationalteration device 8001 serves to increase the etendue of the modifiedradiation beam, when compared to the radiation beam 8200 which isincident on the first diffusing element 8003 a. Due to the uneven natureof the roughened reflective surfaces 8005 a, the direction in which agiven ray of the radiation beam 8001 is output from the radiationalteration device 8001 as part of the modified radiation beam 8007 maybe described by a probability distribution. The probability distributionmay be a continuous function such that the angular intensity profile ofthe modified radiation beam 8007 which is output from the radiationalteration device 8001 is also a continuous function. It may bedesirable to image the angular intensity profile of the modifiedradiation beam 8007 onto a far field plane such that the spatialintensity profile in the far field plane is a continuous function.

The radiation beam 8020 which is incident on the diffusing elements 8003a, 8003 b may be an approximately coherent radiation beam. As wasexplained above, different rays of radiation 8012 which form thecoherent radiation beam 8020 may be reflected in different directions.This may lead to interference occurring between the different rays 8020.Interference between different rays 8020 may lead to a speckle patternoccurring in the modified radiation beam 8012. It may be desirable toreduce or remove the occurrence of speckle in the modified radiationbeam 8012. This may be achieved by rotating the first and/or the seconddiffusing elements 8003 a, 8003 b. Rotation of the diffusing elements8003 a, 8003 b is indicated in FIG. 34 by arrows. The diffusing elements8003 a, 8003 b may, for example, be rotated about central axes of thediffusing elements 8003 a, 8003 b.

Rotation of the diffusing elements 8003 a, 8003 b may advantageouslyreduce the occurrence of speckle in the modified radiation beam 8012.For example, rotation of the diffusing elements 8003 a, 8003 b mayincrease the frequency at which local intensity fluctuations occur inthe modified radiation beam 8012 as a result of interference effects.For example, rotation of the diffusing elements 8003 a, 8003 b mayensure that local intensity fluctuations only occur at frequencies whichare greater than approximately 10 kHz. Local intensity fluctuationswhich occur with frequencies of greater than approximately 10 kHz maynot cause significant problems when all or a portion of the modifiedradiation beam is used in a lithographic exposure process. Localintensity fluctuations which occur with frequencies of greater thanapproximately 10 kHz may therefore be acceptable.

The speed at which the roughened reflective surfaces 8005 a, 8005 b needto move in order to ensure that local intensity fluctuations occur onlyabove a given frequency, may depend on the size of the roughnessfeatures on the roughened reflective surfaces 8005 a, 8005 b (e.g. adiameter d of the indentations 8009). In an embodiment, the roughnessfeatures on the roughened reflective surfaces 8005 a, 8005 b may haveapproximate sizes of about 0.1 mm. In such an embodiment the rotationspeed of the diffusing elements 8003 a, 8003 b may be large enough thatthe roughened reflective surfaces 8005 a, 8005 b move at speeds whichare greater than about 1 meter per second. This may ensure that ensurethat local intensity fluctuations due to interference effects occur atfrequencies above about 10 kHz.

The diffusing elements 8003 a, 8003 b may, for example be forced torotate by one or more actuators (not shown in FIG. 34). Whilst rotationof the diffusing elements 8003 a, 8003 b has been described above, inother embodiments the diffusing elements 8003 a, 8003 b may undergoforms of movement other than rotation. For example, the diffusingelements 8003 a, 8003 b may undergo periodic linear motion in adirection which causes the roughened reflective surfaces 8003 a, 8003 bto remain in approximately the same plane. In general, the diffusingelements 8003 a, 8003 b may be forced to undergo any form of motionwhich ensures that the roughened reflective surfaces 8005 a, 8005 btravel at a speed which is great enough to reduce speckle effects in themodified radiation beam 8007.

Referring again to FIG. 35, the indentations 8009 in the roughenedreflective surface 8005 a have an approximate diameter d. The maximumangle which the roughened reflective surface 8005 a makes with the flatplane 8010 is shown in FIG. 35 and is labelled E. It is desirable thatno rays of radiation 8012 which form the radiation beam 8020 arereflected from the roughened reflective surface 8005 a more than once.In order to ensure that the rays of radiation 8012 are only reflectedonce it is desirable that the magnitude of the maximum angle ε is lessthan about a third of the grazing incidence angle cp. That is, it isdesirable to satisfy the inequality |ε|<φ/3. In some embodiments it maybe desirable for the maximum angle ε to be smaller than this. Forexample, it may be desirable to satisfy the inequality |ε|<φ/3. This mayserve to reduce an asymmetry between rays 8012 which are deflectedupwards or downwards relative to a reflection from the flat plane 8010.

In an embodiment, it may be desirable to generate an angular spread inthe modified radiation beam 8007 of approximately 20 miliradians. Thismay be achieved, for example, with a maximum angle ε of approximately 10miliradians. In some embodiments it may be desirable for the modifiedradiation beam 8007 to have a smaller etendue than will be achieved witha maximum angle ε of approximately 10 miliradians. In such embodimentsthe maximum angle ε may be less than 10 miliradians. For example, themaximum angle ε may be approximately 1 milliradian.

In an embodiment the grazing incidence angle φ may be approximately 70miliradians, the maximum angle ε of approximately 10 miliradians, thediameter d of the indentations 8009 may be approximately 0.1 mm. Theindentations 8009 may have a depth of approximately 250 nm and aradiation of curvature which is approximately equal to 5 mm. If theindentations can be approximated by a portion of a sphere then the depthof the indentations will be approximately equal to dε/4 and the radiusof curvature of the indentations will be approximately equal to d/(2ε).

In some embodiments the grazing incidence angle φ may be less than about5°. In some embodiments the grazing incidence angle φ may be less thanabout 2°. In some embodiments the grazing incidence angle φ may be lessthan about 1°. The use of roughened reflective surfaces which receiveradiation at grazing incidence angles (e.g. grazing incidence angles ofabout 5° or less) advantageously relaxes surface roughness requirementsfor the roughened reflective surfaces. For example, if the roughenedreflective surfaces were to receive radiation at angles of incidencewhich are close to normal incidence then in order to reflect radiationfrom the roughened reflective surface with a given efficiency, thesurface may need to have a surface roughness parameter which is lessthan a given threshold. For an equivalent surface which receivesradiation at grazing incidence angles, the equivalent surface roughnessparameter threshold may be relaxed. For example, the equivalent surfaceroughness parameter threshold may be approximately an order of magnitudegreater for reflection at grazing incidence angles. It may therefore besimpler to manufacture a suitable roughened reflective surface which isconfigured to receive radiation at grazing incidence angles, than it isto manufacture a similar surface to receive radiation at angles ofincidence which are close to normal incidence.

It may be desirable for the above described restrictions on the maximumangle ε to hold true even for small deviations in the surface, which mayhave sizes smaller than the diameter d of the indentations 8009. Forexample, it may be desirable for the restrictions on the maximum angle εto hold true even for any deviations which have a depth which isequivalent to or larger than the wavelength of the radiation beam 8020.A reflective surface may include small scale roughness features (forexample, on a nanometer scale) which form angles which are greater thanthe maximum angle ε.

In general all reflective surfaces include roughness features on somescale. For example, all reflective surfaces include roughness featureson a small scale which is equivalent to or smaller than the wavelengthof the radiation which is being reflected. Reference herein to a“roughened reflective surface” is intended to mean a reflective surfacehaving roughness features which are larger than the wavelength of theradiation which is being reflected and which introduce an angular spreadinto the radiation beam.

In general, it is desirable for the maximum angle ε to be less than halfof a desired angular spread which is to be introduced by the roughenedreflective surface. It is desirable for any roughness features whichhave steeper slopes than this (for example, small scale roughnessfeatures, e.g. on a nanometer scale) to have a height which is less thanabout 5-10 nanometers.

A roughness feature, such as an indentation 8009 as shown in FIG. 35having a diameter d will lead to diffraction angles on the order ofλ/(εφ) where λ is the wavelength of the radiation to be reflected. It isdesirable for the diffraction angles to be small when compared to thedesired angular spread to be introduced by the roughened reflectivesurface. In order for this to be the case the following inequality maybe satisfied d>λ/(εφ). In general, reference herein to a roughenedreflective surface may be interpreted to mean a reflective surfacehaving roughness features whose dimensions are greater than λ/(εφ). Inan embodiment, in which the wavelength λ of the radiation to bereflected is approximately 13 nm, the maximum angle ε is approximately0.01 radians and the grazing incidence angle φ is approximately 0.07radians, then the roughness feature size d may be greater than about 20microns.

In general, a roughened reflective surface may include roughnessfeatures having dimensions which are greater than about 10 microns. Insome embodiments the roughness features may be larger than about 20microns, larger than about 30 microns or larger than about 50 microns.For example, in some embodiments the roughness features may be of theorder of 100 microns.

The roughened reflective surfaces 8005 a, 8005 b may be formed, forexample by roughening a substrate on which a reflective coating isadded. FIG. 36 is a schematic illustration of a cross-section through anembodiment of the first diffusing element 8003 a. The first diffusingelement 8003 a comprises a substrate 8017 and a reflective coating 8019.The substrate 8017 includes an upper surface 8017 a which is roughenedsuch that it includes indentations relative to a flat plane. Thereflective coating 8019 is disposed on the substrate 8017. Theindentations in the uppers surface 8017 a cause the reflective coating8019 to also include indentations. Consequently a roughened reflectivesurface 8005 a is provided.

The indentations in the upper surface 8017 a of the substrate 8017 maybe formed, for example, by abrasive blasting of the upper surface 8017a. The substrate 8017 may, for example, be metallic such that it deformsplastically upon impact from blasting particles. For example, thesubstrate may comprise nickel, copper and/or aluminum. Other methods offorming the indentations may also be used. For example, the uppersurface 8017 a may be impacted with smooth objects such as glass orsteel beads. In some embodiments tumbling may be used to form theindentations in the upper surface 8017 a.

Blasting or tumbling of the upper surface 8017 a may cause the uppersurfaces 8017 a to have a desired macroscopic roughness. However, theupper surface 8017 a may not be smooth enough on a nanometer lengthscale for efficient reflection of radiation (e.g. EUV radiation). Thereflective coating 8019 is therefore disposed on the substrate in orderto provide a reflective surface 8005 a having a desired smoothness on ananometer length scale so as to efficiently reflect radiation (e.g. EUVradiation). The reflective coating 8019 may, for example, be formed fromruthenium. In other embodiments the reflective coating 8019 may, forexample, be formed from molybdenum.

In some embodiments an additional process may be performed in orderincrease the small-scale smoothness of the roughened reflective surface8005 a. For example, an electropolishing process may be performed inorder to smooth the roughened reflective surface 8005 a. Depending onthe materials used to form the substrate and the reflective coating itmay be easier to perform an electropolishing process on the substrate8017, rather than on the reflective coating 8019. In some embodiments,the upper surface 8017 a of the substrate 8017 may therefore besubjected to an electropolishing process prior to disposing thereflective layer 8019 on the substrate.

In an alternative embodiment, a mandrel having desired surfacecharacteristics may initially be formed. The mandrel may, for example,be formed by abrasive blasting of a surface of the mandrel followed byelectropolishing of the surface. The mandrel may be used in anelectroforming process in order to shape an upper surface of a substrate(e.g. a nickel substrate). A reflective coating may then be disposed onthe upper surface of the substrate in order to form a roughenedreflective surface 8005 a.

FIGS. 37A-37E are schematic illustrations of steps of an alternativemethod for forming a diffusing element. FIG. 37A shows a patternedsubstrate 8050 and a metal sheet 8053. The patterned substrate 8050includes protrusions 8051 which extend outwardly from an otherwise flatsurface of the substrate. The protrusions 8051 may be formed, forexample, by a milling process. Alternatively the protrusions may beformed by depositing a coating material onto the substrate 8050. Forexample, a coating material may be deposited onto the substrate using anink-jet printer. The protrusions 8051 may be arranged in a regularpattern. Alternatively, the protrusions 8051 may be randomly distributedacross the surface of the substrate 8050. The shape and/or size of eachof the protrusions 8051 may be approximately the same. Alternatively,the shape and/or size of the protrusions 8051 may be different fordifferent protrusions 8051.

FIG. 37B shows the patterned substrate 8050 being using to deform themetal sheet 8053. For example, the metal sheet 8053 may be deformed bypressing the patterned substrate 8050 and the metal sheet 8053 togetherusing a hydraulic forming process.

The deformed metal sheet 8053 may be used as a mandrel to shape thesurface of a substrate 8055. FIG. 37C shows a substrate 8055 comprisingan upper surface 8057 which is shaped using the deformed metal sheet8053. The upper surface 8057 of the substrate 8055 may, for example, beshaped using an electroforming process.

FIG. 37D shows the shaped substrate 8055 after separation from thedeformed metal sheet 8053. The upper surface 8057 of the substrateincludes indentations 8059. As is shown in FIG. 37E a reflective coating8061 is subsequently added to the upper surface 8057 of the substrate8055 so as to form a roughened reflective surface 8063. The reflectivecoating 8061 may, for example, be formed from ruthenium. Alternativelythe reflective coating 8061 may be formed from molybdenum.

Whilst various methods have been described above for forming a roughenedreflective surface which may form part of a diffusing element, in otherembodiments a roughened reflective surface may be formed using anysuitable method.

A radiation alteration device 8001 of the type shown in FIG. 34 may beused as part of a lithographic system. For example, a radiationalteration device 8001 of the type shown in FIG. 34 may be used tomodify a radiation beam prior to the radiation beam being provided to abeam splitting apparatus. Additionally or alternatively a radiationalteration device 8001 may be used to modify a branch radiation beamprior to the branch radiation beam being provided to a lithographicapparatus.

As was described above, the radiation alteration device 8001 may form amodified radiation beam 8007 having a continuous angular intensityprofile. It may be desirable to image the angular intensity profile ontoa far field plane, which is close to or at a plane at which the modifiedradiation beam is received. For example, it may be desirable to imagethe angular intensity profile onto plane which is close to or at alocation where a beam splitting apparatus is situated, such that thebeam splitting apparatus receives a radiation beam having a continuousspatial intensity profile. Additionally or alternatively it may bedesirable to image the angular intensity profile onto a plane which isclose to or at a location where an optical element of a lithographicapparatus (e.g. a field facet mirror) is situated, such that the opticalelement receives a radiation beam having a continuous spatial intensityprofile.

FIG. 38A is a schematic representation of a focusing scheme which may beused to image the angular intensity profile of a modified radiation beam8007 output from a radiation alteration device 8001 approximately onto afar field plane 8034. The modified radiation beam 8007 is received by afocusing optic 8031. The focusing optic 8031 focuses the modifiedradiation beam so as to pass through an intermediate focus IF. Theintermediate focus IF may, for example, be situated at or near to anopening in an enclosing structure of a lithographic apparatus. Thefocusing optic 8031 also serves to image the modified radiation beamonto a far field plane 8034. The far field plane 8034 may, for example,be a plane in which a field facet mirror of a lithographic apparatus issituated.

Whilst the focusing scheme which is shown in FIG. 38A is represented asbeing formed of transmissive optics, in practice the focusing scheme maybe implemented using reflective optics. The radiation alteration device8001 may, for example, be of the form shown in FIG. 34. The focusingoptic 8031 may, for example, comprise a Wolter telescope formed from aplurality of reflective shells.

In the embodiment which is shown in FIG. 38A, the diameter of theradiation which is incident on the far field plane 8034 may weaklydepend on the beam diameter of the radiation beam 8020 which is receivedby the radiation alteration device 8001. The sensitivity of the diameterof the radiation which is incident on the far field plane 8034 to thediameter of the radiation beam 8020 which is received by the radiationalteration device 8001 may, for example, be reduced by introducing oneor more further optical elements.

FIG. 38B is a schematic representation of an alternative embodiment of afocusing scheme which may be used to image the angular intensity profileof a modified radiation beam 8007 output from a radiation alterationdevice 8001 approximately onto a far field plane 8034. The focusingscheme which is shown in FIG. 38B is similar to the focusing schemewhich is shown in FIG. 38A except that it includes a second focusingoptic 8033 which focuses the modified radiation beam 8007 onto thefocusing optic 8031. The second focusing optic 8033 may serve to reducethe sensitivity of the diameter of the radiation which is incident onthe far field plane 8034 to the diameter of the radiation beam 8020which is received by the radiation alteration device 8001.

In alternative embodiments, the sensitivity of the diameter of theradiation which is incident on the far field plane 8034 to the diameterof the radiation beam 8020 which is received by the radiation alterationdevice 8001, may be reduced by extending the distance between theradiation alteration device 8001 and the far field plane 8034 (forexample, when compared to the distance shown in FIG. 38A). In suchembodiments the size of the focusing optic 8031 may be increased (forexample, when compared with the size of the focusing optic 8031 shown inFIG. 38A).

Whilst the focusing scheme which is shown in FIG. 38B is represented asbeing formed of transmissive optics, in practice the focusing scheme maybe implemented using reflective optics. The radiation alteration device8001 may, for example, be of the form shown in FIG. 34. The focusingoptic 8031 may, for example, comprise a Wolter telescope formed from aplurality of reflective shells.

FIG. 39 is a schematic illustration of an alternative embodiment of aradiation alteration device 9001 according to an embodiment of theinvention. The radiation alteration device 9001 comprises a continuouslyundulating reflective surface 9003. The radiation alteration device 9002is illuminated with a radiation beam. The radiation beam 9002 isrepresented in FIG. 39 by a chief ray 9002 of the radiation beam. In theco-ordinate system used in FIG. 39, the chief ray 9002 is substantiallyparallel with the x-axis. The radiation beam is incident on theradiation alteration device 9001 at a grazing incidence angle cp. Due tothe undulating nature of the surface 9003 different cross-sectionalportions of the radiation beam will be incident on the surface 9003 atdifferent grazing incidence angles. Furthermore the radiation beam mayhave some divergence which results in different rays which form theradiation beam being incident on the surface 9003 at different grazingincidence angles. The grazing incidence angle φ which is shown in FIG.39 is intended to denote the average grazing incidence angle which raysof the radiation beam form with the surface 9003.

The shape of the continuously undulating reflective surface 9003 followsa substantially periodic pattern in both the x and y-directionsindicated in FIG. 39. The continuously undulating reflective surface9003 may be considered to comprise a plurality of reflective portions9005. However, the reflective portions 9005 of a continuously undulatingreflective surface as shown in FIG. 39 differ from, for example, thereflective facets 6103 a-6103 p shown in FIG. 25 or the reflectivefacets 6103′a-6103′m shown in FIG. 26.

The reflective facets 6103 a-6103 p shown in FIG. 25 and the reflectivefacets 6103′a-6103′m shown in FIG. 26 form a reflective surface whichincludes discontinuities. In practice the facets may, for example, beformed from separate elements which are manufactured separately andarranged adjacent to each other. The boundaries of the facets aredefined by sharp edges which form discontinuities in the reflectivesurface provided by the combination of the facets. In contrast, thereflective surface 9003 which is provided by the combination of thereflective portions 9005 shown in FIG. 5 does not include anysubstantial discontinuities. The reflective surface 9003 does nottherefore include any sharp boundaries between adjacent reflectiveportions 9005.

The radiation alteration device which is shown in FIG. 39 may modify aradiation beam in a similar fashion to other embodiments of radiationalteration devices which are described above. For example, the radiationalteration device 9001 may serve to increase the etendue of a radiationbeam. Additionally or alternatively the radiation alteration device 9001may serve to increase the homogeneity of an intensity profile of theradiation beam. The radiation alteration device 9001 may in particularmodify a radiation beam in a similar manner to a radiation alterationdevice comprising a plurality of reflective facets.

In the representation which is shown in FIG. 39, boundaries betweenadjacent reflective portions join points of inflection on the surface.That is, if the extent of the reflective surface 9003 in the z-directionis described by a continuous mathematical function of x and y positions,points of inflection of the continuous function in the x andy-directions define the boundaries between adjacent portions 9005. Eachportion 9005 has a length in the x direction which is substantially halfof a single period of the periodic pattern in the x-direction. Eachportion 9005 has a length in the y-direction which is substantially halfof a single period of the periodic pattern in the y-direction.

The reflective portions 9005 shown in FIG. 39 may be divided into threedifferent classes of reflective portion. A first class of reflectiveportion 9005 a may be referred to as a convex portion 9005 a. A convexportion 9005 a has a positive curvature in both the x and y-directions.That is, the second derivative with respect to x and y of a continuousfunction describing the surface 9003 as a function of x and y remainspositive throughout a convex portion 9005 a. A second class ofreflective portion 9005 b may be referred to as a saddle portion 9005 b.A saddle portion 9005 b has a positive curvature in one of the x andy-directions and a negative curvature in the other of the x andy-directions. A third class of portion 9005 c may be referred to as aconcave portion 9005 c. A concave portion 9005 c has a negativecurvature in both the x and y-directions.

The portions 9005 may be configured such that each portion 9005 receivesa cross-sectional portion of the radiation beam which is substantially asquare. Since the radiation beam propagates substantially parallel withthe x-axis and is incident on the radiation alteration device 9001 at arelatively small grazing incidence angle φ, the extent of the portions9005 in the x-direction is greater than the extent of the portions inthe y-direction. In other embodiments the cross-sectional portion of theradiation beam which is incident on each portion 9005 may have a shapeother than a square. For example, the cross-sectional shape may besubstantially rectangular.

As was described above, the reflective portions 9005 shown in FIG. 39may be formed from a single reflective surface 9003. Such a surface maybe easier to manufacture than a plurality of reflective portions formedfrom separate elements, such as for example the reflective portions 6103a-6103 p shown in FIG. 25 and the reflective portions 6103′a-6103′mshown in FIG. 26. Reflective portions which are manufactured separatelyfrom each other and then positioned adjacent one another may bedifficult to position such that radiation is not lost at the boundariesbetween portions. For example, gaps may exist between adjacent portionswhich may lead to a loss of radiation at the gaps. The continuouslyundulating reflective surface 9003 shown in FIG. 39 may be manufacturedwithout any gaps in between adjacent portions such that no radiation islost at boundaries between portions.

FIG. 40 is a schematic illustration of a unit cell of a continuouslyundulating surface 9003 which may form a portion of a radiationalteration device 9001 according to an embodiment of the invention. Theunit cell 9007 comprises a single period P_(x) of the undulating surface9003 in the x-direction and a single period P_(y) of the undulatingsurface 9003 in the y-direction. The unit cell 9007 comprises a convexportion 9005 a, a concave portion 9005 c and two saddle portions 9005 b.

The aspect ratio of the portions

$\frac{P_{y}}{P_{x}}$may be selected such that each portion 9005 receives an approximatelysquare shaped cross-sectional portion of the incident radiation beam.For example, the aspect ratio may be approximately equal to sin(φ)≈φ,where φ is the average grazing incidence angle with which radiation isincident on the reflective surface 9003.

The extent of the surface 9003 in the z-direction (which may be referredto as the height of the surface) may be expressed as a continuousmathematical function of x and y. The function may be expressed as:z(x,y)=ƒ(x)+g(y)  (6)where ƒ(x) is a periodic function of x having a period P_(x) and g(y) isa periodic function of y having a period P_(y).

In the representation shown in FIG. 40, the origin of the x and y-axes(i.e. where x=0 and y=0) is chosen to correspond with the geometriccenter (on the x and y-axes) of a portion 9005 (in this case the convexportion 9005 a). If the origin of the x and y-axes correspond with thecenter of a portion then the reflective surface may satisfy thefollowing symmetry relations:ƒ(x+½P _(x))=−ƒ(x)  (7)ƒ(x)=ƒ(−x)  (8)g(x+½P _(x))=−g(x)  (9)g(x)=g(−x).  (10)

In some embodiments the surface 9003 may be defined such that withineach portion 9005 z(x,y) is a second order function of both x and y.That is, the curvature of the surface 9003 may be substantially constantin both the x and y-directions within each portion 9005. In such anembodiment, when each portion is illuminated with radiation it willreflect the incident radiation so as to illuminate an approximatelyrectangular portion of a far field location. If a portion is illuminatedwith radiation having a substantially homogenous spatial intensityprofile then the approximately rectangular portion of the far fieldlocation which is illuminated by radiation reflected from the portion9005 will also have a substantially homogenous spatial intensityprofile. A portion 9005 may, for example, be illuminated with radiationhaving a substantially homogenous spatial intensity profile if the sizeof the portion 9005 is small compared to any spatial variation in theintensity profile of a radiation beam incident on the radiationalteration device 9001.

In other embodiments, the surface 9003 may be defined such that thecurvature of the surface 9003 is different at different positions withina portion. The cross-sectional shape and the spatial intensity profileof radiation reflected from the surface 9003 may, for example, becontrolled by controlling the curvature of the surface 9003 within theportions 9005.

In some embodiments, in which each portion 9005 is arranged to receive aportion of the cross-section of a radiation beam having a substantiallysquare shape of width and height a, the expressions ƒ(x) and g(x) inequation (6) may given by the following equations:

$\begin{matrix}{{f(x)} = {\frac{a\;\sigma_{m}}{4\;\varphi}Z\mspace{11mu}( \frac{2\;{\sin(\varphi)}x}{a} )}} & (11) \\{{g(y)} = {\frac{a\;\sigma_{m}}{4\;\varphi}Z\mspace{11mu}( \frac{2\; y}{a} )}} & (12)\end{matrix}$where Z is a dimensionless function of the term inside the brackets inequations (11) and (12), φ is the average grazing incidence angle atwhich the radiation beam is incident on the surface 9003 and σ_(m) isthe maximum angle by which radiation is deflected by the reflectivesurface 9003. The angles φ and σ_(m) are in radians. The terms insidethe brackets in equations (11) and (12) are dimensionless and varybetween −1 and 1 over the extent of a single portion 9005.

FIG. 41A is a schematic representation of an embodiment of the functionZ as a function of X, where X is the term inside the brackets inequation (11). Also shown in FIG. 41A is the resulting function ƒ(x)given by equation (11) as a function of x. FIG. 41B is a schematicrepresentation of the first derivative of the functions shown in FIG.41A. A similar form of the function Z may be used to define g(y) inequation (12).

The form of the function Z which is shown in FIGS. 41A and 41B is merelyan example embodiment. In the example shown in FIGS. 41A and 41Bradiation reflected from the reflective surface 9003 will have anapproximately Gaussian angular intensity profile. In the depictedexample, the Gaussian distribution is cut off at approximately ±2.5standard deviations. In other embodiments other cut-off points may bechosen.

In other embodiments the shape of the reflective surface 9003 may bedefined so as to produce other forms of angular intensity profile. Forexample, a desired intensity profile and shape in a far field locationmay be chosen and an appropriate shape of the reflective surface 9003which achieves the desired intensity profile and shape may be determinedand manufactured. In general choosing a wider angular distribution willlead to smaller curvature on the surface 9003 especially in they-direction and may result in relatively large values of the secondderivative of g(y). In general it may be desirable to use relativelysmall values of the grazing incidence angle φ since this will lead to areduced loss of EUV radiation at the reflective surface. However, themaximum angular deflection σ_(m) is less than the grazing incidenceangle φ. The grazing incidence angle φ may therefore be chosen to besufficiently large to achieve a desired maximum angular deflectionσ_(m).

FIG. 42 is a schematic illustration of a radiation alteration device9001 of the type described above with reference to FIGS. 39-41. A chiefray 9002 of a radiation beam is shown being incident on a radiationalteration device 9001 at a grazing incidence angle φ. Whilst notvisible in FIG. 42, the radiation alteration device 9001 has acontinuously undulating reflective surface 9003 of the form describedabove with reference to FIGS. 41A and 41B. The resulting shape of thereflected radiation beam in a far field plane 9009 is also shown in FIG.42.

FIG. 42 is a schematic representation of the normalized angularintensity distribution in the far field plane 9009 shown in FIG. 42.Contour lines shown in FIG. 43 indicate normalized intensity valuesbetween 0 and 1 in steps of 0.1. The highest intensity values are in thecenter of the representation and the intensity decreases with increasingdistance from the center. In the example shown in FIG. 43, the grazingincidence angle φ was chosen to be approximately 70 milliradians (mrad)and the maximum angular deflection σ_(m) was chosen to be approximately45 milliradians. In the depicted example the ratio φ/σ_(m) is thereforeapproximately 0.64. In other embodiments the ratio φ/σ_(m) may beselected to be greater than or less than 0.64.

The angular intensity profile which is shown in FIG. 43 is merely anexample of an angular intensity profile which may be formed by aradiation alteration device 9001 comprising a continuously undulatingreflective surface 9003. In some embodiments it may be desirable to forman angular intensity profile of the type shown in FIG. 43 in which theintensity is at a maximum at a center of the profile and decreases withincreasing radial distance from the center. However in other embodimentsit may be desirable to form an angular intensity profile of a differenttype. The shape of the reflective surface 9003 (e.g. the curvature ofthe surface 9003) may be changed in order to change the angularintensity profile formed by radiation reflected from the surface 9003. Adesired angular intensity profile may be dependent on the configurationof a lithographic apparatus LA which is arranged to receive theradiation beam which is modified by the radiation alteration device9001.

In the embodiment which is shown in FIG. 43, the full extent of theangular intensity profile may not be used by a lithographic apparatusLA. For example, a lithographic apparatus may be arranged to receive adisc shaped intensity profile. It will be appreciated that a disc shapedintensity profile may be achieved simply be discarding corners of theintensity profile (for example, by blocking the propagation of cornersof the intensity profile).

As was described above with reference to other embodiments of aradiation alteration device, it may be desirable to configure aradiation alteration device 9001 such that a resulting angular intensityprofile is relatively insensitive to changes in the position and/or thediameter of the radiation beam which is incident on the radiationalteration device. In general, decreasing the size of the portions 9005which form the radiation alteration device 9001 will generally decreasethe sensitivity of the reflected intensity profile to changes in theposition and/or the diameter of the radiation beam which is incident onthe radiation alteration device 9001.

As was described above a lithographic apparatus may be configured tocapture and use a disc shaped portion of the angular intensity profilewhich is provided by the radiation alteration device 9001. The power ofradiation which is captured by the lithographic apparatus may vary withvariations in the position and/or diameter of the radiation beam whichis incident on the radiation alteration device 9001. Furthermore thecentroid of the intensity distribution which is captured by thelithographic apparatus LA may vary. In general decreasing the size ofthe reflective portions 9005 and therefore increasing the number ofreflective portions 9005 which form a radiation alteration device 9001,may decrease the variations in the captured power and/or the position ofthe centroid of the intensity distribution. The number of portions 9005and the size of the portions 9005 may be chosen in order to provide adesired level of stability of the captured power and/or the centroidposition.

In some embodiments the radiation beam which is incident on theradiation alteration device 9001 may have an intensity profile whichapproximately follows a Gaussian distribution. The diameter of theradiation beam may be considered to be the diameter of four standarddeviations of the Gaussian distribution. Such a diameter may be denotedD_(4σ). A ratio of the diameter D_(4σ) to the portion period P_(y) inthe y-direction may be denoted M=D_(4σ)/P_(y). In some embodiments itmay be desirable for the shift in position of the centroid of the outputintensity distribution to be less than about 10% of the radius of thedisc which is captured by a lithographic apparatus. In some embodimentsthis may for example be achieved if M is greater than about 1.3. Theshift in centroid may be less than about 1% of the radius of the disc ifM is greater than about 2. The shift in centroid may be less than about0.1% of the radius of the disc if M is greater than about 7. The aboverelations may, in some embodiments, be used in order to select the sizeof the portions 9005. The above relations may apply for mathematicallyperfect surfaces. For real surfaces which are subject to manufacturingtolerances the values of M may be increased in order to achieve thedesired results.

As was described above with reference to other embodiments of radiationalteration devices, a modified radiation beam which is output from aradiation alteration device 9001 may be imaged by one or more focusingoptics. FIG. 44A is a schematic representation of a focusing schemewhich may be used to image a modified radiation beam which is outputfrom a radiation alteration device 9001 approximately on to a far fieldplane 9034. The representation which is shown in FIG. 44A is a paraxialrepresentation similar to the paraxial representations shown in FIGS.12, 32 and 38.

The radiation alteration device 9001 outputs a modified radiation beam9011. The modified radiation beam 9011 is received by a focusing optic9013. The focusing optic 9013 focuses the modified radiation beam so asto pass through an intermediate focus IF. The intermediate focus IF may,for example, be situated at or near to an opening in an enclosingstructure of a lithographic apparatus. The focusing optic 9013 alsoserves to image the modified radiation beam 9011 onto a far field plane9034. The far field plane 9034 may, for example, be a plane in which afield facet mirror of a lithographic apparatus is situated.

Whilst the focusing scheme which is shown in FIG. 44A is represented asbeing formed of transmissive optics, in practice the focusing scheme maybe implemented using reflective optics. The radiation alteration device9001 may, for example, be of the form shown in FIG. 39. The focusingoptic 9013 may, for example, comprise a Wolter telescope formed from aplurality of reflective shells. Alternatively the focusing optic 9013may comprise a reflective surface having an approximately ellipsoidalshape. Alternatively the focusing optic 9013 may comprise a plurality ofreflective elements. For example, the focusing optic 9013 may comprise aWolter telescope formed from two reflective elements.

In the embodiment which is shown in FIG. 44A, the diameter of theradiation which is incident on the far field plane 9034 may weaklydepend on the beam diameter of the radiation beam which is received bythe radiation alteration device 9001. The sensitivity of the diameter ofthe radiation which is incident on the far field plane 9034 to thediameter of the radiation beam 8020 which is received by the radiationalteration device 8001 may, for example, be reduced by introducing oneor more further optical elements.

In the embodiment of FIG. 44A, shifts in the position of the radiationbeam which is incident on the radiation alteration device 9001 may leadto shifts in the position of the radiation incident on the far fieldplane 9034.

FIG. 44B is a schematic representation of an alternative embodiment of afocusing scheme which may be used to image the angular intensity profileof a modified radiation beam 9011 output from a radiation alterationdevice 9001 approximately onto a far field plane 9034. The focusingscheme which is shown in FIG. 44B includes a first focusing optic 9013 aand a second focusing optic 9013 b. The first focusing optic 9013 a isarranged to focus the angular intensity profile of the modifiedradiation beam 9011 onto an image plane 9015. The second focusing optic9013 b is arranged to focus the image plane 9015 onto the far fieldplane 9034 via an intermediate focus IF. Since the image plane 9015 isimaged onto the far field plane 9034, the focusing scheme serves toimage the angular profile of the modified radiation beam 9011 outputfrom the radiation alteration device 9001 onto the far field plane 9034.The intermediate focus IF may, for example, be situated at or near to anopening in an enclosing structure of a lithographic apparatus. The farfield plane 9034 may, for example, be a plane in which a field facetmirror of a lithographic apparatus is situated.

In the embodiment which is shown in FIG. 44B the image plane 9015 issituated in between the first focusing optic 9013 a and the secondfocusing optic 9013 b. In alternative embodiments the image plane 9015may be a virtual image plane and may, for example, be situated inbetween the second focusing optic 9015 and the far field plane 9034. Inthe representation which is shown in FIG. 44B radiation in the imageplane 9015 has roughly the same diameter as radiation incident on thefirst focusing optic 9013 a. However, in other embodiments the diameterof radiation in the image plane 9015 may be smaller or larger than thediameter of the radiation incident on the first focusing optic 9013 a.

Whilst the focusing scheme which is shown in FIG. 38B is represented asbeing formed of transmissive optics, in practice the focusing scheme maybe implemented using reflective optics. The first and/or the secondfocusing optics 9013 a, 9013 b may, for example, comprise a Woltertelescope formed from a plurality of reflective shells. Alternativelythe first and/or second focusing optics 9013 a, 9013 b may comprise areflective surface having an approximately ellipsoidal shape.Alternatively the first and/or the second focusing optics 9013 a, 9013 bmay comprise a plurality of reflective elements. For example, the firstand/or second focusing optics 9013 a, 9013 b may comprise a Woltertelescope formed from two reflective elements.

The focusing schemes shown in FIGS. 44A and 44B may be used inconjunction with any of the other embodiments of radiation alterationdevices described herein. Furthermore any of the other embodiments offocusing schemes described herein may be used in conjunction with aradiation alteration device of the type described with reference toFIGS. 39-43.

Embodiments have been described throughout the description in which amodified branch radiation beam is imaged onto a far field plane throughan intermediate focus IF. The intermediate focus IF may be situated ator near an opening 8 in an enclosing structure of a lithographicapparatus LA. However, in some embodiments a branch radiation beam maynot be focused through an intermediate focus IF. FIG. 45 is a schematicillustration of an alternative embodiment of a lithographic apparatusLA_(a)′ which includes a relatively large opening 8 in an enclosingstructure. The lithographic apparatus LA_(a)′ which is shown in FIG. 45is similar to the lithographic apparatus LA_(a) shown in FIG. 2. Likefeatures in FIGS. 2 and 45 are provided with like reference numerals andthe corresponding features will not be described in detail again withreference to FIG. 45.

The lithographic apparatus LA_(a)′ which is shown in FIG. 45 includes anopening 8 for accepting a branch radiation beam B_(a) which is largerthan the corresponding opening 8 which is shown in FIG. 2. Consequently,in the embodiment of FIG. 45 the branch radiation beam B_(a) need not befocused to an intermediate focus IF in order for the branch radiationbeam B_(a) to be able to pass into the lithographic apparatus LA_(a)′. Afocusing scheme which is used to image a modified branch radiation beamneed not therefore focus a modified branch radiation beam B_(a) throughan intermediate focus IF.

A lithographic apparatus LA_(a)′ which is configured to accept a branchradiation beam B_(a) which has not been focused through an intermediatefocus IF, may include a field facet mirror 10′ which is modified whencompared to a field facet mirror 10 of a lithographic apparatus which isconfigured to accept a branch radiation beam B_(a), which has not beenfocused through an intermediate focus IF (e.g. the field facet mirror 10of the lithographic apparatus of FIG. 2). For example the modified fieldfacet mirror 10′ may include reflective facets having different focallengths when compared to the field facet mirror 10 of the lithographicapparatus LA_(a) of FIG. 2. Additionally or alternatively theorientation of the reflective facets may be different in the modifiedfield facet mirror 10′.

As was described above, for example with reference to FIGS. 13-29 aradiation system may comprise a beam splitting apparatus configured tosplit a main radiation beam into a plurality of branch radiation beams.FIG. 46 is a schematic illustration of a beam splitting apparatus 9050according to an embodiment of the invention. The beam splittingapparatus is configured to receive a main radiation beam and split themain radiation beam into a plurality of branch radiation beams. The beamsplitting apparatus may be configured to receive the main radiation beamalong a beam axis 9051. The beam axis extends into the page of FIG. 46.The beam splitting apparatus comprises a plurality of reflective facets9053. A radiation beam which is incident on the beam splitting apparatus9050 illuminates a plurality of reflective facets 9053 and may forexample, illuminate substantially all of the reflective facets 9053shown in FIG. 46.

The reflective facets 9053 form a plurality of groups 9055 of reflectivefacets 9053. For example, a first group of reflective facets 9055 a isshown in FIG. 46 with blocks filled with diagonal lines. A second group9005 b of reflective facets 9053 is shown in FIG. 46 with blocks filledwith cross-hatching. A third group 9005 c of reflective facets 9053 isshown in FIG. 46 with blocks filled with black dots on a whitebackground. A fourth group 9005 d of reflective facets 9053 is shown inFIG. 46 with blocks filled with white dots on a black background. Theother reflective facets 9053 which form the beam splitting apparatus9050 form further groups of reflective facets. However, for ease ofillustration the further groups of reflective facets are notspecifically indicated in FIG. 46.

Each group of reflective facets 9055 comprises a plurality of reflectivefacets 9053 arranged to receive different portions of a radiation beam.The reflective facets 9053 which form a single group 9055 of reflectivefacets 9053 are arranged to reflect the different portions received bythe different facets 9053 in a group 9055 so as to form a branchradiation beam comprising a combination of the different reflectedportions. That is, each group 9055 of reflective facets forms a singlebranch radiation beam from radiation reflected from the facets 9053which form that group 9055. For example, the first group of reflectivefacets 9055 a reflects portions of a radiation beam so as to form afirst branch radiation beam (not shown in FIG. 46). The branch radiationbeam comprises a combination of the portions of the radiation beamreflected from the facets 9053 which make up the first group of facets9055 a.

Reflective facets 9053 which form a group of facets 9055 may all havesubstantially the same orientation. Consequently radiation which isreflected from the facets which from the group of facets 9055 propagatesin substantially the same direction such that it forms a single branchradiation beam. Different groups of facets 9055 may be orientateddifferently such that different groups of facets 9055 reflect radiationin different directions so as to form different branch radiation beams.

As can be seen in FIG. 46, different reflective facets 9053 which form agroup of reflective facets 9055 are situated in different positions in across-section of a radiation beam. That is a plurality of reflectivefacets 9053 which form a group of facets 9055 are separated from eachother in a direction which is substantially perpendicular to the beamaxis 9051. A branch radiation beam which is formed from reflections fromreflective facets which form a group of reflective facets will thereforeinclude different portions of the cross-section of the radiation beamwhich is incident on the beam splitting apparatus 9050.

It may be desirable for each branch radiation beam which is formed by abeam splitting apparatus 9050 to have substantially the same power. Forexample, each branch radiation beam may be supplied to a differentlithographic apparatus and it may be desirable to provide eachlithographic apparatus with radiation of substantially the same power.The division of the reflective facets 9053 into groups of reflectivefacets may be selected such that the total power of radiation which isincident on each group of reflective facets is substantially the same.Consequently each resulting branch radiation beam may have substantiallythe same power.

A radiation beam which is incident on the beam splitting apparatus 9050may have a cross-sectional intensity profile which is rotationallysymmetric about the beam axis 9051. For example, the cross-sectionalintensity profile of the radiation beam may be approximated by atwo-dimensional Gaussian. That is, the cross-sectional center of theradiation beam (which may coincide with the beam axis 9051) may have thehighest intensity of radiation. The intensity of radiation may decreasewith increasing radial distance from the cross-sectional center. Thedecrease of the intensity with increasing radial distance maysubstantially follow a Gaussian distribution.

As is shown in FIG. 46 the arrangement of reflective facets 9053 (in theplane of the page, which is perpendicular to the beam axis 9051) whichform the second group of facets 9055 b is substantially the same as arotation (about the beam axis 9051) of the arrangement of the reflectivefacets 9053 which form the first group 9055 a of reflective facets. Thecross-sectional portion of the radiation beam which is reflected by thesecond group of reflective facets 9055 b, and which forms a secondbranch radiation beam, is thus a rotation (about the beam axis 9051) ofthe cross-sectional portion of the radiation beam which is reflected bythe first group of reflective facets 9055 a, and which forms a firstbranch radiation beam. If the cross-sectional intensity profile of theradiation beam which is incident on the beam splitting apparatus 9050 issubstantially rotationally symmetric (about the beam axis 9051), thenthe total power of radiation which is incident on the first group 9055 aof facets will be substantially the same as the total power of radiationwhich is incident on the second group 9055 b of facets. The power of thefirst branch radiation beam will therefore be substantially the same asthe power of the second branch radiation beam.

In the embodiment of FIG. 46 each group of reflective facets 9055comprises an arrangement of reflective facets 9053 which issubstantially the same as a rotation of an arrangement of reflectivefacets 9053 which forms another group of reflective facets 9055.Furthermore, each group of reflective facets 9055 comprises anarrangement of reflective facets 9053 which is substantially the same asa different rotation of an arrangement of reflective facets 9053 whichforms every other group of reflective facets 9055. The rotation is arotation about the beam axis 9051. If the cross-sectional intensityprofile of the radiation beam which is incident on the beam splittingapparatus 9050 is substantially rotationally symmetric (about the beamaxis 9051), then the total power of radiation which is incident on eachgroup of facets will be substantially the same. The power of each branchradiation beam will therefore be substantially the same.

References herein to an arrangement of reflective facets 9053 which forma group of reflective facets 9055 is intended to refer to a positioningof the reflective facets in the path of the radiation beam. For example,the arrangement of reflective facets 9053 which form a group ofreflective facets 9055 may refer to a positioning of the reflectivefacets in a cross-section of the radiation beam. References to anarrangement of facets being substantially the same as a rotation ofanother arrangement of facets is merely intended to refer to thepositioning of the facets and not to the orientation of the facets. Thatis, a rotation of a first group of facets may result in facets whichhave a different orientation to a second group of facets such that therotation of the first group of facets may reflect radiation in adifferent direction to the second group of facets. However, if therotation of the first group of facets results in facets in substantiallythe same position as the second group of facets then the arrangement ofthe second group of facets may still be considered to be substantiallythe same as a rotation of the arrangement of the first group of facets.

The facets 9053 which form a group of reflective facets 9055 may besituated at substantially the same location along the beam axis 9051.Furthermore the different groups of reflective facets 9055 may besituated at substantially the same location along the beam axis 9051.References herein to facets being situated at substantially the samelocation along the beam axis 9051 is not intended to mean that each offacets 9053 is situated in the same plane but merely that there is nolarge separation between the facets along the beam axis 9051.

As can be seen in FIG. 46, some of the reflective facets 9053 fromdifferent groups of reflective facets 9055 are situated adjacent to eachother. Some of the reflective facets 9053 from different groups ifreflective facets 9055 may be in contact with each other. The facets9053 which form the beam splitting apparatus 9050 may, for example, forma single piece of apparatus. This may assist in manufacturing the beamsplitting apparatus and may assist in aligning and orientating thereflective facets 9053 relative to the radiation beam. For example, thealignment and orientation of the reflective facets 9053 may becontrolled by controlling the alignment and orientation of the beamsplitting apparatus as a whole.

In some embodiments the relative positioning and orientation of thefacets 9053 which form the beam splitting apparatus may be fixed. Inother embodiments the relative orientation of the facets 9053 which formthe beam splitting apparatus 9050 may be adjustable. For example, thebeam splitting apparatus may comprise one or more actuators which areoperable to alter the orientation of one or more of the facets 9053.

In the embodiment which is shown in FIG. 46, each group of reflectivefacets 9055 is formed from six different facets. The beam splittingapparatus 9050 comprises eight different groups of reflective facets9055 which will therefore result in eight different branch radiationbeams. In other embodiments a beam splitting apparatus may be formedfrom more or less than eight groups of reflective facets. Each group ofreflective facets may comprise more or less than six different facets.

The arrangement of reflective facets which form different groups ofreflective facets shown in FIG. 46 is merely an example of a possiblearrangement. Other arrangements of facets which form groups of facets asdescribed above may instead be used and may result in the same orsimilar advantageous effects described above in connection with theembodiment of FIG. 46.

FIGS. 47A-47C are schematic illustrations of alternative embodiments ofa beam splitting apparatus 9050. Each beam splitting apparatus 9050comprises a plurality of reflective facets 9053 organized into groups ofreflective facets 9055. The groups of reflective facets 9055 have thesame properties which were described above with reference to FIG. 46 buta distributed about the beam splitting apparatuses 9050 differently. Inthe representations shown in FIGS. 47A-47C facets 9053 which belong tothe same group of facets 9055 are provided with the same shading. Forease of illustration not all of the different groups of reflectivefacets in FIGS. 47A-47C are labelled.

FIG. 48 is a schematic illustration of a further seven possible layoutsof reflective facets 9053 which may form a beam splitting apparatus 9050comprising a plurality of groups of reflect facets 9053 as describedabove. In the embodiments which are shown in FIG. 48, the arrangement offacets 9053 into groups of facets 9055 is not shown. However, it will beappreciated that the facets 9053 which form any of the embodiments shownin FIG. 48 may be arranged to form groups of facets as was describedabove with reference to FIG. 48.

A beam splitting apparatus 9050 of the form described above withreference to FIGS. 46-48 may be used in conjunction with a radiationalteration device described herein. For example, one or more of thebranch radiation beams provided by the beam splitting apparatus 9050 maybe modified with a radiation alteration device. Additionally oralternatively a main radiation beam may be modified with a radiationalteration device prior to the main radiation beam being incident on abeam splitting apparatus 9050.

Various embodiments of a radiation alteration device comprising aplurality of reflective facets have been described above. A radiationalteration device comprising a plurality of reflective facets may beused similarly to a radiation alteration device comprising a tube havingfirst and second openings (as was described with reference to FIGS.1-23). For example, a radiation alteration device comprising a pluralityof reflective facets may be arranged to receive and modify a branchradiation beam B_(a) prior to the branch radiation beam B_(a) beingprovided to a lithographic apparatus LA. Additionally or alternatively aradiation alteration device comprising a plurality of reflective facetsmay be arranged to receive a main radiation beam B from a radiationsource SO prior to the beam radiation beam B being provided to a beamsplitting apparatus.

A radiation alteration device comprising a plurality of reflectivefacets may, in some applications, provide one or more advantages whencompared to a radiation alteration device comprising a tube having areflective internal surface. For example, a radiation alteration devicecomprising a plurality of reflective facets may occupy less space than aradiation alteration device comprising a tube. This may in particularsimplify the arrangement of a lithographic system in which a radiationalteration device is provided to modify each of a plurality of branchradiation beams, since the total space which is occupied by radiationalteration devices may be significantly reduced.

Additionally or alternatively a radiation alteration device whichcomprises a plurality of reflective facets may be easier and/or cheaperto manufacture and/or clean than a radiation alteration devicecomprising a tube having a reflective internal surface. For example, thetotal surface area of reflective facets which form a radiationalteration device may be less than the total surface area of areflective internal surface which forms a radiation alteration device.Consequently it may be easier and/or cheaper to manufacture and/or cleana radiation alteration device comprising a tube having a reflectiveinternal surface.

The reflective facets which form a radiation alteration device may, forexample, be manufactured separately and combined into a single element.The reflective facets may be manufactured by electroforming a reflectivematerial onto a substrate. For example, the reflective facets may bemanufactured by electroforming nickel onto a substrate.

Whilst embodiments of a radiation source SO have been described anddepicted as comprising a free electron laser FEL, a radiation source SOmay include a source of radiation other than a free electron laser FEL.

It should be appreciated that a radiation source which comprises a freeelectron laser FEL may comprise any number of free electron lasers FEL.For example, a radiation source may comprise more than one free electronlaser FEL. For example, two free electron lasers may be arranged toprovide EUV radiation to a plurality of lithographic apparatus. This isto allow for some redundancy. This may allow one free electron laser tobe used when the other free electron laser is being repaired orundergoing maintenance.

A lithographic system LS may comprise any number of lithographicapparatus. The number of lithographic apparatus which form alithographic system LS may, for example, depend on the amount ofradiation which is output from a radiation source SO and on the amountof radiation which is lost in a beam delivery system BDS. The number oflithographic apparatus which form a lithographic system LS mayadditionally or alternatively depend on the layout of a lithographicsystem LS and/or the layout of a plurality of lithographic systems LS.

Embodiments of a lithographic system LS may also include one or moremask inspection apparatus MIA and/or one or more Aerial InspectionMeasurement Systems (AIMS). In some embodiments, the lithographic systemLS may comprise a plurality of mask inspection apparatuses to allow forsome redundancy. This may allow one mask inspection apparatus to be usedwhen another mask inspection apparatus is being repaired or undergoingmaintenance. Thus, one mask inspection apparatus is always available foruse. A mask inspection apparatus may use a lower power radiation beamthan a lithographic apparatus. Further, it will be appreciated thatradiation generated using a free electron laser FEL of the typedescribed herein may be used for applications other than lithography orlithography related applications.

It will be further appreciated that a free electron laser comprising anundulator as described above may be used as a radiation source for anumber of uses, including, but not limited to, lithography.

The term “EUV radiation” may be considered to encompass electromagneticradiation having a wavelength within the range of 4-20 nm, for examplewithin the range of 13-14 nm. EUV radiation may have a wavelength ofless than 10 nm, for example within the range of 4-10 nm such as 6.7 nmor 6.8 nm.

The lithographic apparatus which have been described herein may be usedin the manufacture of ICs. Alternatively, the lithographic apparatusesdescribed herein may have other applications. Possible otherapplications include the manufacture of integrated optical systems,guidance and detection patterns for magnetic domain memories, flat-paneldisplays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

Different embodiments may be combined with each other. Features ofembodiments may be combined with features of other embodiments.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the claims set out below.

The invention claimed is:
 1. A radiation alteration device comprising: areflective surface having a non-discontinuous undulating profile,wherein the undulating profile of the reflective surface follows asubstantially periodic pattern in a first direction and in a seconddirection perpendicular to the first direction, wherein the period ofthe pattern in the first direction is different than the period of thepattern in the second direction, and wherein the reflective surface isconfigured to reflect a majority of incident extreme ultraviolet (EUV)radiation; and an optical element configured to direct a radiation beamat the reflective surface such that an average angle of the radiationbeam incident on the reflective surface is a grazing angle of 8.5degrees or less, the radiation beam being the highest intensityradiation beam to be provided incident on the reflective surface at onetime.
 2. The radiation alteration device of claim 1, wherein a unit cellof the periodic undulating reflective surface comprises: a first portionhaving a substantially convex shape; a second portion having asubstantially concave shape; a third portion having a substantiallysaddle shape; and a fourth portion having a substantially saddle shape.3. The radiation alteration device of claim 2, wherein the unit cellcomprises a single period of the periodic pattern in a first directionand a single period of the periodic pattern in a second directionperpendicular to the first direction.
 4. The radiation alteration deviceof claim 2, wherein the reflective surface is shaped such that, withinone or more selected from the at least one of the first, second, thirdand/or fourth portions, the reflective surface has a curvature isdefined at least in part by a second-order polynomial.
 5. The radiationalteration device of claim 2, wherein the reflective surface is shapedsuch that, within one or more selected from the at least one of thefirst, second, third and/or fourth portions, a curvature of thereflective surface is different at different positions in the respectiveportion.
 6. The radiation alteration device of claim 1, wherein thereflective surface is configured to receive a radiation beam and reflectthe radiation beam so as to form a modified radiation beam and whereinthe reflective surface is shaped such that the modified radiation beamhas an intensity distribution in a far field plane, the intensitydistribution comprising an intensity maximum value, wherein intensityvalues in the intensity distribution decrease with increasing radialdistance from the intensity maximum value.
 7. A radiation systemcomprising: a radiation source configured to emit EUV radiation; theradiation alteration device of claim 1 configured to receive a radiationbeam comprising at least a portion of the EUV radiation emitted by theradiation source.
 8. A lithographic system comprising: a radiationsystem according to claim 7; and a lithographic apparatus arranged toreceive at least a portion of an EUV radiation beam which exits aradiation alteration device.
 9. The lithographic system of claim 8,wherein the lithographic apparatus includes an illumination systemconfigured to condition at least a portion of the EUV radiation beamwhich exits the radiation alteration device, the illumination systemincluding a facet mirror and wherein the radiation system comprises atleast one focusing optic configured to focus the EUV radiation beamwhich is provided to the lithographic apparatus so as to form an imageof the radiation beam output from the radiation alteration device ontothe facet mirror, and wherein the facet mirror comprises a plurality ofreflective facets.
 10. A method, comprising: providing a radiation beamto a reflective surface of a radiation alteration device such that anaverage angle of the radiation beam incident on the reflective surfaceis a grazing angle of 8.5 degrees or less, the radiation beam being thehighest intensity radiation beam incident on the reflective surface atone time and the surface having a non-discontinuous undulating profile,wherein the undulating profile of the reflective surface follows asubstantially periodic pattern in a first direction and in a seconddirection perpendicular to the first direction, wherein the period ofthe pattern in the first direction is different than the period of thepattern in the second direction, and wherein the reflective surface isconfigured to reflect a majority of incident extreme ultraviolet (EUV)radiation; and outputting a modified radiation beam from the surface.11. The method of claim 10, wherein a unit cell of the periodicundulating reflective surface comprises: a first portion having asubstantially convex shape; a second portion having a substantiallyconcave shape; a third portion having a substantially saddle shape; anda fourth portion having a substantially saddle shape.
 12. The method ofclaim 11, wherein the unit cell comprises a single period of theperiodic pattern in a first direction and a single period of theperiodic pattern in a second direction perpendicular to the firstdirection.
 13. The method of claim 11, wherein the reflective surface isshaped such that, within one or more selected from the at least one ofthe first, second, third and/or fourth portions, the reflective surfacehas a shape is defined by a second-order polynomial.
 14. The method ofclaim 11, wherein the reflective surface is shaped such that, within oneor more selected from the at least one of the first, second, thirdand/or fourth portions, a curvature of the reflective surface isdifferent at different positions in the respective portion.
 15. Themethod of claim 10, wherein the reflective surface is shaped such thatthe modified radiation beam has an intensity distribution in a far fieldplane, the intensity distribution comprising an intensity maximum value,wherein intensity values in the intensity distribution decrease withincreasing radial distance from the intensity maximum value.
 16. Themethod of claim 10, wherein the radiation beam comprises EUV radiation.17. The method of claim 10, further comprising receiving at least of aportion of the modified beam from the radiation alteration device to aportion of a lithographic apparatus.
 18. The method of claim 17, furthercomprising: conditioning at least a portion of the modified radiationbeam which exits the radiation alteration device using a facet mirror,wherein the facet mirror comprises a plurality of reflective facets; andfocusing the at least portion of the modified radiation beam so as toform an image of the radiation beam output from the radiation alterationdevice onto the facet mirror.